题干
—The air pollution is terrible.
— It will be worse we take action to protect the environment.
A：if
B：unless
C：so
D：though
答案(点此获取答案解析）
B
—The air pollution is terrible.
— It will be worse we take action to protect the environment.
A：if
B：unless
C：so
D：though
，则在此过程中发出的光子总数是：