London 2012's Olympic torch relay started( )May 19

答案 D 解释如下： at, in , on, 和by 表示时间的区别： at指时间表示： 1）时间的一点,一时刻： They come home at surprise (at noon, at midnight, at ten o’clock, at daybreak, at dawn)。 2) 较短的一段时间。可指某个节日或认为一年中标志大事的日子。 His grandfather died at seventy-two。 At the beginning (middle, end) of that month he was sick。 He went home at Christmas (at New Year, at the spring Festival, at night, at midsummer)。In指时间表示：1）在某个较长的时间（如世纪,朝代,年,月,季节,及泛指的上午,下午和晚上等）内。 He died in the holidays。 He was born in 1942。2) 在一段时间之后。He will come round in a day or two。I will finish it in three hours。On指时间表示：1）具体的时日和一个特定的时间,如某日,某节日,星期几等。He will come to meet us on our arrival。On May 4th (On Sunday, On New Year’s day, On Christmas Day), there will be a celebration。2) 在某个特定的早晨,下午或晚上。He arrived at 10 o’clock on the night of the 5th。He died on the eve of victory。3) 准时,按时。If the rain should be on time, I should reach home before dark。