题干
I had difficulty _______ the maths problem.
A：working out
B：work out
C：to work out
D：works out
答案(点此获取答案解析）
A
I had difficulty _______ the maths problem.
A：working out
B：work out
C：to work out
D：works out
沿中垂线运动，忽略重力作用，则（ ）