问两道sat语法题1.Many changes occurred while she was president of

问两道sat语法题
1.Many changes occurred while she was president of the ( ).
括号里面可以是A(college;these changes increased its educational quality and effectiveness)
B(college;these changes increased both the educational quality and effectiveness of the college)
看起来A更简洁,但是答案是B.
2.In Germany,foresters discovered that trees killed by acid rain had begun to die four years ( )
括号里面可以是A(earlier without any signs of disease down shown then)
B(earlier,even though the trees had shown no signs of disease then)
看起来同样是A更简介可是答案同样是B
求详细解答以及这种类型错误的一般应对方法,