Teachers should ___them to continue studying when some students want to give it up.

A．encourage

B．keep

C．prevent

D．make

植树造林是造福后人的千秋大业，春季植树时总是将大量的侧枝除去，只留很少的枝芽，这样做的目的主要是为了减小树的_______________，从而减少树的水分______________，提高树的成活率。