题目选项分析

By sucking sap from the young twigs of the hemlock tree, tree growth is retarded by the woolly adelgid, and this causes the color of needles to change from deep green to grayish green, and their dropping prematurely.

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.