请教OG10 207

207. In the textbook publishing business, the second
                quarter is historically weak, because revenues are low and marketing expenses are high as companies prepare for the coming school year.

(A)            low and marketing expenses are high as companies prepare

(B)            low and their marketing expenses are high as they prepare

(C)            low with higher marketing expenses in preparation

(D)            low, while marketing expenses are higher to
                prepare

(E)            low, while their marketing expenses are higher in
                preparation

正确答案是A，我想问一下A选项里面的and明明就是连接两个

句子啊，为什么中间不加逗号呢？谢谢。