单选题17.The touch excites no defensive response unless the approach is from above where the spider can see the motion, ______on its hind legs, lifts its front legs, opens its fangs and holds this threatening posture as long as the object continues to move.
【正确答案】
A
【答案解析】本句中空白处表达的意思是：在何种情况下它举起它的后腿。in which case指代的是前面句中所述情况，无须倒装，因此本题答案为A。