今天看到一個PP的邏輯題，驚覺GMAC竟然用了數種比較結構在句中，剖析一下:
Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas.
第一種是S+V+O結構的比較級，比主格為了避免主受格歧意問題，補出助動詞以明確化比較對象。
In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouses at their schools and filtered the greenhouse air to eliminate sulfur dioxide.
Plants in the urban greenhouse grew more slowly than those in the rural greenhouse.
第二種是用those的結構，雖然前面也是S+V+O結構的比較級，但是因為用those讀者不會有主受格歧意問題，所以再補出助動詞就違反修辭的省略原則。
Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas.
第一種是S+V+O結構的比較級，比主格為了避免主受格歧意問題，補出助動詞以明確化比較對象。
In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouses at their schools and filtered the greenhouse air to eliminate sulfur dioxide.
Plants in the urban greenhouse grew more slowly than those in the rural greenhouse.
第二種是用those的結構，雖然前面也是S+V+O結構的比較級，但是因為用those讀者不會有主受格歧意問題，所以再補出助動詞就違反修辭的省略原則。
