Animals should not be locked in cages; they should be in their normal .
A. environment
B. position
C. residence
D. dwelling
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(答案→)A. environment
B. position
C. residence
D. dwelling
解析： environment(=surroundings, circumstances)环境，environment作集合名词用时，只用单数形式：We should work hard to improve our environment. (单词，不可数)Children need a happy home environment. (单数，可数)