A．buses are always available in 24 hours B．bus riders have to buy monthly passes C．bus fare is paid by exact change in coins D．buses are the best means of transportation
单项选择题4（）
A．box B．book C．glass D．paper
单项选择题Plateau s experiments were the first step leading to the invention of（）
A．eyeglasses B．telescopes C．bifocals D．motion pictures
