题目选项分析

By sucking sap from the young twigs of the hemlock tree, the woolly adelgid retards tree growth, causing needles to change color from deep green to grayish green and to drop prematurely.

By sucking sap from the young twigs of the hemlock tree, the woolly adelgid retards tree growth, which causes needles to change color from deep green to grayish green, and dropping prematurely.