The fact has worried many scientists the earth is becoming warmer and warmer these years.

问题补充：

The fact has worried many scientists the earth is becoming warmer and warmer these years.

A. what

B. which

C. that

D. though

答案：

A. what

B. which

(答案→)C. that

D. though

解析：考查同位语从句，that后的从句解释前面的名词The fact，句意：这些年来地球变得越来越暖和这个事实使科学家很担忧。所以选C。