■ bow and arrow

“弓箭”如果理解为一种武器的整体，作为主语时动词用单数。

• The bow and arrow was invented at that time.弓箭是那个时候发明的。

• twice
• twin
• ukraine
• un-
• unaware
• unawares
• unaware 与 unawares
• under
• under-
• underestimate
• undersell
• unhuman
• unilateral
• unilateral 与 one-sided
• uninterested
• union
• unite
• united states
• university
• unless
• unlike
• unload
• unsatisfied
• until
• until