But for friction (摩擦力), people neither walk, nor even stand up.
A. can
B. were able to
C. will be able to
D. could
网友回答
A. can
B. were able to
C. will be able to
(答案→)D. could
解析：
考点分析：虚拟语气。要不是摩擦力，人们即不能走也不能站立。典型的虚拟语气，but for 要不是的意思，是此题目的关键来判定是虚拟语气。故选 D
考点 考查虚拟语气