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:

There is no hard and fast rule regarding this subject.

A rule that is difficult

B rule that is fat changing

C rule that cannot be broken or modified

D rule that can be broken or modified

ANS:C - rule that cannot be broken or modified

No answer description is available.