The plane will take off at 6:00 pm, so I have to be at the airpo

The plane will take off at 6:00 pm, so I have to be at the airport 4:00 pm at the latest.
A、 after
B、 until
C、 since
D、 by
【正确答案】：D
【题目解析】：by 4:00表示“在四点之前”。四个选项只有by有这个意思。