问题：
[单选题]关于树干用线的说法正确的是
The right way of line making in drawing the outline of tree is?
A照抄树干的全部细节
draw the whole details of branch
B用尽可能长的直线切出外轮廓
use straight line as much as possible
C用笔要反复勾描
trace the outline multiple times
D灵活运用线条，以线条表达树干的扭转及遮挡关系
use line to show different layers and motion relationship of branches
