大学英语单项选择题每日一练(2019.09.18)

单项选择题
If you（）bright sunlight（）dry wood with a glass, it will start burning.
A.expose；to
B.concentrate；on
C.aim；at
D.focus；on
单项选择题
Food alone is not the（）of energy.
A.source
B.resource
C.force
D.grocer
单项选择题
We agreed to accept（）they thought was the best tourist guide.
A.whatever
B.whomever
C.whichever
D.whoever
单项选择题
It was（）of you not to play the piano while I was having a sleep.
A.considerate
B.careful
C.considering
D.considerable
单项选择题
Economics, several courses of which I have taken thus far,（）to be difficult but useful for almost all students。
A.prove
B.havebeenproved
C.proves
D.areproved