求 老prep 82题 分词形式

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82. (28967-!-item-!-188;#058&003792)
Each year companies in the United States could save as much as $58 billion annually by preventing illness among employees and gain as much as $200 billion through improving performance of workers if they simply provided offices with cleaner air.
(A) annually by preventing illness among employees and gain as much as $200 billion through improving performance of workers if they simply provided
(B) annually if they prevented employee illness and gain as much as $200 billion through worker performance improved by simply providing
(C) annually in employee illness prevention and gain as much as $200 billion through worker performance improved by simply providing
(D) in employee illness prevention and gain as much as $200 billion through improving performance of workers if they simply provided
(E) by preventing illness among employees and gain as much as $200 billion through improved worker performance if they simply provided
答案上说，现在分词含有进行的意思，过去分词含有完成的意思。所以A不用improving，因为performance已经被提高，才造成收益。但是，前半句中节省不是也已经形成？为什么用preventing，不是也表示正在进行？

1030430882

求高人指点，我也不懂。。