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. The single biggest gainer in this process
2. was ITC's Gold Flake Kings sales are estimated
3. to have moved up from 50 million to 200 million sticks per month during 1987 and last year.
4. No error.