Question

Fill in the blanks with a suitable modal verb :
You ______ be mad to do this.
a. Shall
b. Must
c. Can
d. Could

Answer 1

Hint: To answer the question, one must understand proper usage of modal verbs. It is important to know which one is used in what context.

Complete answer:
Verbs are the part of speech that shows an action being taken place. There are various forms of verbs depending on the subject and the tense. Walk, talk, be, swim are some verbs. Example, “I have been running for the past thirty minutes”. Modal verbs are auxiliaries that express necessity or possibility. Example, shall, will, can, could, etc.
In the given sentence, “you must be mad to do this”, is the only way the sentence can make sense, because the other modals ‘can’ and ‘could’ are talking about capabilities, which is not the case here. ‘shall’ cannot be used either, because it does not make sense in the context and is usually used with the pronoun “I”.

Option (a), “shall”, as discussed above, cannot be used in this context. Hence, it is an incorrect option. 

Option (b), “must” is the correct modal to be used here, since it is the only one that dictates the definiteness in the context, as discussed above, hence, option b is the correct.

Option (c), “can”, makes no sense in the given context, since it talks about capabilities, hence it is an incorrect choice.

Option (d), “could”, is irrelevant for the same reason as the above option, hence it is incorrect as well. Option (d) is the correct choice.

Hence option (b) and option (d) is correct.

Note: Here, one must know the proper usage of forms of modal verbs in order to not confuse the options. If one is not aware, one might end up choosing the wrong option.