---------------------------------------以下是问题---------------------------------------- 但是prep185 (GWD-8-Q41)的非画线部分是:When drive-ins were at the height of their popularity in the late 1950s, some 4,000 existed in the United States, but today there are less than one-quarter that many.
这里我的理解是:1.逻辑主语=句子主语=drieve-ins, 2. 为be动词结构。那么为什么这里可以这么写呢?是因为这个是“同留”么? 如果是,那么“所以when a child/ when children是错的。。。”为什么是错的?