桁架结构形式与载荷Fp均已知，结构中杆件内力为零的杆件数为：（）。
A.零根
B.2根
C.4根
D.6根
参考答案：A
In fact, ________ focus, but also we should have it at every level of our life.[ ]
A. not only we should have
B. not only should we have
C. we not only should have
D. not only should have we
土壤的物理性质主要包括哪些方面？它们对作物有何影响？
