练习：
1.A therefore B however C although D whatever
2.A shelters B foods C disasters D water
3.A missed B protected C raised D caught
4.A such B too C so D even
5.A feel B see C hear D sense
6.A waves B tides C winds D rivers
7.A birthplaces B playground C reserve D storage
8.A experimental B apparent C scientific D chemical
9.A specialist B assistant C supporter D sponsor
10.A additional B specific C especial D exceptional
11.A modification B detection C assessment D value
12.A route B behavior C principle D phenomenon
13.A unwillingly B occasionally C doubtfully Dcertainly
14.A some B much C many D few
15.A on B as C for D in