a variety of句中动词单复数
问题描述：
a variety of句中动词单复数
A wide variety of opinions have been expressed on the plausibility of these threats.
和A considerable variety of opinions on this subject has at different times been advanced from the earliest times.
都是 a variety of opinions为主语,为什么一句用单数一句用复数?
这个是《英语语法实践指南第四版》主谓一致里面的。书上说通常a variety of+复数名词+复数动词，后面又写a variety of也见于这样的句中（第二句）。第二句到底特殊在哪里了？非要让a variety of 这么用？
答
语法上来说,应该用复数