自行车是由一堆自行车零件组装成具有交通功能的工具。这个系统属于()
A.自然系统和人造系统
B.实体系统和抽象系统
C.自然系统和社会系统
D.人造系统和实体系统
参考答案:D
The research mainly deals with the difficulty the students have _____ their attention on their study all the time. [ ]
A. to focus
B. to be focused
C. focusing
D. focused
燃料喷射装置由哪些组成?