练习：
1.A attack B meet C love D visit
2.A terrible B eatable C valuable D possible
3.A source B existence C friends D fish
4.A Space B Sky C Land D Earth
5.A because B since C because of D by reason that
6.A whose B which C that D where
7.A to B for C like D with
8.A times B places C seas D oceans
9.A sciences B mathematics C chemicals D physics
10.A feelings B senses C touches D tastes
11.A those B these C another D other
12.A systems B processes C ideas D circles
13.A recover B reform C return D rely
14.A rivers B oceans C forests D mountains
15.A weak B little C few D great