Old buses are major （ ） to air pollution in this city. (contribute)
A、
contribute
B、
contribution
C、
contributors
D、
contributing
【正确答案】：C
【题目解析】：
由are可知此空应填名词复数形式。
Old buses are major （ ） to air pollution in this city. (contribute)
Old buses are major （ ） to air pollution in this city. (contribute)
contribute
contribution
contributors
contributing
由are可知此空应填名词复数形式。