单选题
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】解析：根据空格处的意思“在何种情况下它举起它的后腿”，可知in which case是对前面情况的指代，不引起倒装，故选A。