People cut down too many trees the greenhouse effect becomes more and more serious.A. so

问题补充：

People cut down too many trees, the greenhouse effect becomes more and more serious.

A. so

B. because

C. although

D. if

答案：

(答案→)A. so

B. because

C. although

D. if

解析：so 因此；because 因为；although虽然，尽管； if是否，如果。句意：人们砍伐了太多的树木，因此温室效应变得越来越严重。结合语境可知选A。