Meaning: 'Main' means 'most important' or 'primary,' indicating that protecting organs is a key function of bones.

Wrong options:

• first: 'First' implies a sequence rather than importance, which does not fit the context of discussing functions.
• stable: 'Stable' suggests a state of being steady or unchanging, which does not convey the idea of importance in this context.
• important: While 'important' conveys significance, it is less specific than 'main' and does not fit as smoothly in the sentence structure.

Correct answer: the

Explanation: The article 'the' is necessary here to specify the heart and lungs as particular organs being protected, fitting grammatically and contextually within the sentence.

Meaning: 'The' is a definite article used to refer to specific items, in this case, the heart and lungs, indicating they are known entities in the context.

Wrong options:

• then: 'Then' indicates a sequence in time and does not fit grammatically or contextually in this sentence.
• there: 'There' is used to indicate a place or existence, which does not apply here as we are discussing specific organs.
• this: 'This' is a demonstrative pronoun that refers to something nearby, which does not fit the context of referring to the heart and lungs.

Correct answer: three

Explanation: The word 'three' fits grammatically as it specifies the number of types of joints, aligning with the context of categorizing joints in the skeletal system.

Meaning: 'Three' indicates a specific quantity, meaning there are exactly three types of joints.

Wrong options:

• several: 'Several' is vague and does not specify an exact number, which is necessary in this context.
• many: 'Many' also suggests a large but unspecified number, failing to provide the precise count needed here.
• three: This option is not incorrect but is the same as the correct answer, thus not applicable.

Correct answer: and

Explanation: The conjunction 'and' appropriately connects the last item in a list of joint types, fitting grammatically and contextually to indicate addition.

Meaning: 'And' is used to connect words or phrases, indicating that freely moving joints are included in the list of joint types.

Wrong options:

• or: 'Or' suggests an alternative rather than addition, which does not fit the context of listing joint types.
• because: 'Because' introduces a reason, which is not relevant in this context of listing items.
• hence: 'Hence' indicates a conclusion or result, which does not apply to the context of simply listing types.