Identify the segment in the sentence which contains a grammatical error: 'Mathematics are a very difficult subject for those who do not work hard.'

Identify which part of the sentence has an error: These kind of trousers/ (A) are rather/ (B) expensive to buy./ (C) No error (D)

Find the part of the sentence that has a grammatical error: 'The citizens of the area longed on rain, which is natural when it is so terribly dry year-round.'