A:频率50Hz、周期0.2秒
B:频率50Hz、周期0.02秒
C:频率50kHz、周期0.02秒
D:频率100Hz、周期0.01秒
-- Have you sent him a birthday present?
-- . I gave it to him two days ago
下列句子中,划线成语使用正确的一项是( )
—_____ He didn't even spend a single hour preparing for it.