Meaning: A 'boom' refers to a period of rapid growth or increase, particularly in economic or construction activities.

Wrong options:

• ratio: The term 'ratio' refers to a quantitative relationship between two numbers, which does not fit the context of construction growth.
• rate: While 'rate' is related to speed or frequency, it does not convey the idea of a significant increase in construction activity as 'boom' does.
• prosperity: 'Prosperity' refers to a state of being prosperous or successful, which does not specifically relate to construction activity.
• mechanized: 'Mechanized' is an adjective describing something that is operated by machines, which does not fit as a noun in this context.
• applied: 'Applied' is a verb that does not serve as a noun and does not fit the grammatical structure needed here.
• expected: 'Expected' is an adjective that does not convey the idea of growth or increase in construction.

Correct answer: mechanized

Explanation: The term 'mechanized' is an adjective that describes the tunnelling techniques as being operated by machines. This fits grammatically and contextually as it specifies the type of techniques being discussed.

Meaning: 'Mechanized' refers to processes that are operated or controlled by machinery rather than manual labor.

Wrong options:

• ratio: 'Ratio' is a noun that does not describe a type of tunnelling technique and is irrelevant in this context.
• rate: 'Rate' is a noun that refers to a measure or quantity, which does not describe the techniques themselves.
• prosperity: 'Prosperity' is a noun that refers to success or wealth, which does not relate to tunnelling techniques.
• boom: 'Boom' is a noun that describes growth but does not describe the nature of the tunnelling techniques.
• applied: 'Applied' is a verb that does not fit as an adjective to describe techniques.
• expected: 'Expected' is an adjective that does not describe the nature of tunnelling techniques.

Correct answer: applied

Explanation: The word 'applied' is the past participle of the verb 'apply,' fitting grammatically as it indicates that the techniques have been utilized in practice. This aligns with the context of discussing their use in construction.

Meaning: 'Applied' means to have put something into practical use or action, particularly in a specific context.

Wrong options:

• ratio: 'Ratio' does not fit as it is a noun and does not relate to the action of using techniques.
• rate: 'Rate' is a noun that does not convey the action of utilizing techniques in construction.
• prosperity: 'Prosperity' is a noun that does not relate to the action of applying techniques.
• mechanized: 'Mechanized' is an adjective and does not fit as a verb in this context.
• boom: 'Boom' is a noun that does not relate to the action of applying techniques.
• expected: 'Expected' is an adjective that does not convey the action of utilizing techniques.

Correct answer: rate

Explanation: The word 'rate' fits grammatically as a noun that describes the speed or frequency of excavation. It aligns with the context of discussing the efficiency of tunnelling techniques.

Meaning: 'Rate' refers to the speed at which something occurs, in this case, the speed of excavation in tunnel construction.

Wrong options:

• ratio: 'Ratio' refers to a quantitative relationship and does not fit the context of excavation speed.
• prosperity: 'Prosperity' is a noun that refers to success or wealth, which does not relate to excavation speed.
• mechanized: 'Mechanized' is an adjective that does not fit as a noun in this context.
• applied: 'Applied' is a verb that does not relate to the measurement of excavation speed.
• boom: 'Boom' refers to growth but does not specifically relate to the speed of excavation.
• expected: 'Expected' is an adjective that does not convey the idea of speed or frequency.