单项选择题
A. is able to design new tools B. measures the amount of energy used C. enables people to see the muscular movements D. visualizes electrical signals
A．you buy it B．you write a letter C．put a letter into a postbox D．it has a postmark on it
A．used for phones, bus fares and hotel reservations B．used to pay in restaurants and big stores C．used like credit cards, even in remote areas D．exchanged easily, even at car rental companies