We've just installed two air-conditioners in our apartment, ______ should make great differences in our life next summer.
【正确答案】 A
【答案解析】逗号后是非限制性定语从句，需要填入引导词，故A项which正确。B项what和C项that都不能引导非限制性定语从句；D项they能充当后一分句的主语，但两个分句间无连词，语法上同样不正确。