Question

Select the appropriate modal auxiliary to complete the sentence:

They don't have the key so they______ go inside the house.

• A. couldn't
• B. can't
• C. mustn't
• D. won't

Answer 1

Answer: (B)

can't

Modal Verbs show us the attitude of the speaker to what is being said or done. The term "modal" means expressing mood and mood is a way to express the attitude of the speaker.

The given sentence is firstly set in present tense and secondly according to the meaning of the sentence it expects a mood of impossibility.

Option B is the correct answer because it expresses impossibility or inability in the past.

Whereas option A expresses inability but in the past form;

option D expresses impossibility in future;

Option C expresses something which is not supposed to happen.