第2题也选 are。即句子的谓语动词与 gallons 保持一致，不与 water 保持一致。

Hundreds of thousands of people are at risk. 有几十万人正处于危险中。

Millions of gallons of water are kept back by the dam. 这个堤坝拦住了滔滔的洪水。

又如：

Hundreds of tons of rubbish were moved. 搬运了数百吨垃圾。

Thousands of acres of forest were on fire. 数千英亩森林起火了。

Thousands of tons of steel were shipped out from the docks. 数千吨的钢铁从码头运走了。