Meaning: 'Shelters' means to provide protection or cover for something, in this case, the heart and lungs.

Wrong options:

• displaces: Displaces means to move something from its place, which does not convey the protective function intended in this context.
• envelopes: Envelopes suggests wrapping something completely, which is not the precise action of the ribs as they provide a cage-like structure rather than a complete wrap.
• hurts: Hurts indicates causing pain, which is the opposite of the intended meaning of protection in this context.

Correct answer: two

Explanation: The word 'two' is the correct numeral that indicates the quantity of bones meeting at a joint. It fits grammatically as a cardinal number and contextually as it accurately describes the typical number of bones that form a joint, aligning with the passage's focus on anatomy.

Meaning: 'Two' refers to the number 2, indicating that typically two bones come together at a joint.

Wrong options:

• to: To is a preposition that does not indicate quantity and does not fit the context of counting bones.
• too: Too is an adverb meaning excessively, which does not apply in this context where a specific number is needed.
• too many: Too many is a phrase indicating an excessive quantity, which contradicts the straightforward count of bones at a joint.