[求助]OGl(紫色) SC-Q11 关于frequency的疑问

有个小疑问：

It is well known in the supermarket industry that how items are placed on shelves and the frequency of inventory turnovers can be crucial to profits.

平行结构，所以正确的应该是：E. how frequently the inventory turns over can be. 答案显而易见～

选项B. the frequency of inventory turnovers is often 我们不管它是否和前面平行，单看它的主谓语一致。为什么is不对呢？ frequency不是单数吗？

求教！！谢谢！

什么意思？我还是不理解。

ls的能不能举个例子？谢谢！