-- Where is dad, mum?
-- He is ____his report in the room. He needs it for a meeting.
A:looking like
B:looking at
C:looking for
D:looking after
小力同学加热2mL蒸馏水,操作的正确顺序是____ (填番号).
①用试管夹夹好试管 ②点燃酒精灯 ③加热
④盖好试剂瓶,放回原处 ⑤往试管中加入2mL蒸馏水.
在托里拆利实验中,下面哪种情况能使玻璃管内外水银面的高度差发生变化