It's difficult to live in a foreign country, ______ if you can't understand the language there.
A:exactly
B:naturally
C:usually
D:especially
如图所示,用手拨动塑料尺,发出的声音是由塑料尺____产生的。塑料尺振幅越大,声音的____越大。若改变塑料尺伸出桌面的长度,会使声音的____发生改变。