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. Since the attachment of airconditioned sleeping cars to all important trains,
2. travelling became very pleasant,
3. especially during the summer season.
4. No error.