Old buses are major ______ to air pollution in this city. （contr

Old buses are major ______ to air pollution in this city. （contribute）
【正确答案】：contributions
【题目解析】：此处需要填入作表语的词，且前面被形容词major修饰，故要用名词，同时还要注意此处需要用名词的复数形式。