问题补充：
How this kind of plant can be in a special way is still unknown to us.
A. kept away
B. kept out
C. kept alive
D. kept off
答案：
A. kept away
B. kept out
(答案→)C. kept alive
D. kept off
解析：keep alive意为:使…活着; keep away意为:使…离开; keep out意为:不使入内,防止; keep off意为:不让接近.
