液压系统中产生爬行的原因可能为（）。
A、液压油进了空气
B、液压油乳化
C、液压油含杂质多
参考答案：A
—Are _____ their books?
—Yes, _____ are.[ ]
A. these; their
B. those; their
C. this; they
D. those; they
