Directions: Read the sentence to find out whether there is any grammatical or idiomatic error in it. The error, if any, will be in one part of the sentence. The number of that part is the answer. If there is no error, the answer is (d). (Ignore the errors of punctuation, if any)

I meet him (a)/ once a blue moon (b)/ so I do not know much about his activities. (c)/ No error. (d)/