Cloning an animal is a plex that can go fatally wrong at any stage.
A. process
B. regulation
C. step
D. method
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(答案→)A. process
B. regulation
C. step
D. method
解析：这题考查词义辨析：process过程，regulation规定，step步骤， method方法，根据句意：克隆动物是个复杂的过程，每一步都可能出现致命的错误。选A。