What __________ they putting on the Christmas tree?[ ]
A. was
B. is
C. are
答案：C
试述坐骨神经痛的最常见病因，主要的症状及体征，以及主要的保守治疗方法。
A chain bridle is used when towing astern because it（）.
A.is easy to connect
B.provides an effective catenary and absorbs shock due to its weight
C.makes rigging a swivel unnecessary
D.prevents the tow from yawing by the drag of the chains in a seaway
