英语18.To reach the top of the building ,you have to climb 18 _______ of _______.

英语18.To reach the top of the building ,you have to climb 18 _______ of _______.
18.To reach the top of the building ,you have to climb 18 _______ of _______.
a.flight;stairs b.flights;stairs c.flight;stair d.flights;stair
为什么