What did Tom do last night?
A:He did his homework.
B:He watched TV
C:He drew some pictures.
我国冬季南北气温相差很大的主要原因是( )
香料化学家已经开发出多种脂类人工香料,如丁酸甲酯(化学式为C5H10O2)具有苹果香味。下列有关丁酸甲酯的说法正确的是