这是一种什么症状
A．躁狂症状B．牵连观念C．焦虑症状D．强迫症状E．强制性思维
参考答案：D
There are four birds ______ the tree. And there're many oranges ____ it, too. [ ]
A. in, in
B. in, on
C. on, in
D. on, on
前角和后角根据工件材料的软硬与否，可取正值，也可取负值．
