语法题：SAT OG中的　improving sentence
The convenience and availability of watercolor paint (account for its popularity) with amateur artists.括号中的东西需要改进吗?
A. 不改变
B.account for their popularity
C. accounts for its popularity
D. is why it is popular
E. are a reason for its popularity
答案选A.解释上说,the verb "account" correctly refers back tothe plural subject "convenience and availability". 但是,convenience and availability 不是两个不可数名词吗?两个不可数名词放在一起,谓语动词就变复数了吗?那什么情况下,谓语动词才是单数呢?