our earth or else it will be no longer fit for us to live on.A. ProtectB. To protectC. Pr

问题补充：

our earth, or else it will be no longer fit for us to live on.

A. Protect

B. To protect

C. Protecting

D. Protected

答案：

(答案→)A. Protect

B. To protect

C. Protecting

D. Protected

解析：考查特殊句式。祈使句+ and / or 后用一般将来时态，句意：保护好地球，否则它将不再适合我们在上面生活。所以选A。