Read each sentence to find out whether there is any grammatical error in it. The error, if any will be in one part of the sentence. The letter of that part is the answer. If there is no error, the answer is 'D'. (Ignore the errors of punctuation, if any).

1. Last month we celebrated
2. the wedding of our sister for whom
3. we have been looking for a suitable alliance for three years.
4. No error.