题目

Trees should only be pruned（修剪）when there is a good and clear reason for doing so and, the number of such reasons is small. Pruning involves the cutting away of overgrown and unwanted branches, and the inexperienced gardener can be encouraged by the thought that more damage results from doing it unnecessarily than from leaving the tree to grow in its own way.      Firstly, pruning may be done to make sure that trees have a desired shape or size. The object may be to get a tree of the right height, and at the same time to help the growth of small side branches, which will th“神七”太空舱利用NiFe2O4做催化剂将宇航员呼出的CO2转化为O2，己知Fe的化合价为+3，则Ni的化合价为A.+1B.+2C.+3D.+4