Hello,<br/><br/>1. They boarded the train at quarter to six o'clock.<br/>I have to correct the above sentence. I know that "o'clock " is not required here.<br/><br/>But I want know that-<br/>2. They boarded the train <b>at</b> quarter to six.<br/>3. They boarded the train <b>at a</b> quarter to six. <br/>Which one is correct 2 or 3?<br/>Thanks.