Idioms and Phrases - Verbal and Reasoning

In the following questions four alternatives are given for the idiom/phraseitalicised and underlinedin the sentence. Choose the alternative which best expresses the meaning of idiom/phrase.
Q1:

He is an interesting speaker but tends to go off at a tangent.

A change the subject immediately

B forget things in between

C go on at great length

D become boisterous

ANS:A - change the subject immediately

No answer description is available.