—Which is more useful, reading or writing?
—I think _______ of them are useful.
A:either
B:both
C:all
D:each
如下图,摆1个梯形需要4根小棒,摆2个梯形需要7根小棒…照这样摆下去,摆10个梯形,需要____根小棒,46根小棒可以摆____个这样的梯形。