Meaning: 'Blocks' refers to the essential components or units that make up a larger structure, in this case, the fundamental elements of life.

Wrong options:

• cells: 'Cells' refers specifically to biological units and does not convey the broader concept of foundational components in this context.
• spans: 'Spans' suggests a range or extent, which does not align with the idea of basic components of life.
• designs: 'Designs' implies a plan or arrangement, which is not suitable for describing the fundamental units of life.

Correct answer: extra-terrestrial

Explanation: The term 'extra-terrestrial' is the correct choice as it specifically refers to life forms that originate outside of Earth, fitting the context of discussions about life beyond our planet.

Meaning: 'Extra-terrestrial' means originating, existing, or occurring outside the Earth or its atmosphere, particularly in reference to life forms.

Wrong options:

• alien: 'Alien' can refer to foreign entities but lacks the specific scientific context of life beyond Earth that 'extra-terrestrial' provides.
• native: 'Native' implies origin from a particular place, which contradicts the idea of life from outside Earth.
• celestial: 'Celestial' refers to heavenly bodies or space but does not specifically denote life forms, making it less precise in this context.

Correct answer: triggers

Explanation: The word 'triggers' is appropriate as it indicates an action that initiates or causes a response, fitting the context of what activates the life-mechanism.

Meaning: 'Triggers' means to cause something to happen or to initiate a process, particularly in a biological context.

Wrong options:

• imperials: 'Imperials' refers to something related to an empire or authority, which does not fit the context of biological mechanisms.
• evokes: 'Evokes' suggests bringing something to mind or calling forth, which is not as precise as 'triggers' in describing an initiating action.
• twigs: 'Twigs' refers to small branches and is unrelated to the concept of initiating biological processes.

Correct answer: span

Explanation: The word 'span' fits grammatically as a noun indicating a period or duration of time, which aligns with the context of how long a simple living system can develop into a complex species.

Meaning: 'Span' refers to the extent or duration of time over which something occurs, particularly in developmental contexts.

Wrong options:

• test: 'Test' implies an examination or assessment, which does not fit the context of time duration for development.
• gap: 'Gap' suggests a break or interval, which does not convey the continuous process of development implied in the sentence.
• sequence: 'Sequence' refers to a specific order of events, which does not accurately represent the idea of time duration in this context.

Correct answer: exist

Explanation: The word 'exist' is the correct choice as it denotes the state of being present or alive, which is essential in the context of contacting advanced species.

Meaning: 'Exist' means to have actual being or presence, particularly in reference to life forms.

Wrong options:

• hold: 'Hold' implies possession or retention, which does not fit the context of life forms being present.
• desire: 'Desire' refers to wanting or wishing for something, which is not relevant to the existence of species.
• die: 'Die' indicates the cessation of life, which is contrary to the context of discussing the presence of advanced civilizations.