首先肯定，你的选择是正确的。

英语中，像hundred, thousand, million, billion这些数词，可用来表示“具体”的数字，也可以用来表示“大概”（不确切）的数字。例如：三百人：three hundred people；数百人：hundreds of people。

表示具体数目，之前加one (a), two, three, seven等具体数字：one (two, three, seven) hundred people，之后不带of。

若表示“大概”（不确切）的数字，数词本身用复数形式，后跟of，其前不能有具体的数字修饰，例如：

hundreds of workers

thousands of cows

tens of thousands of trees

但是，这些数词前被some, several, a few 等词修饰时，两种用法均可。可能是为了语句简练，这些不具体的数字当做具体数字用。

several thousand houses = several thousands of houses

a few hundred policemen = a few hundreds of policemen

some million trees = some millions of trees

several thousands of houses 的例句：

The oil well extended several hundreds of feet in depth.  油井向下延伸了数百英尺。

It has been one of China's best-loved novels for several hundreds of years.  这是中国几百年来最令人喜爱的小说之一。

Several hundreds of office workers in that company walked off their jobs in a wage dispute.  那家公司的几百名雇员因工资纠纷而进行罢工。

There are several hundreds of such places still discoverable in the track of the great war.  我们循着大战的痕迹，可以找到成百上千这样的地方。