单项选择题
A．and B．so C．but D．or
A．illustrate some experiments with old people B．introduce more information about the infants C．devote some space to the background information about the experiment D．talk about different theories on learning
A．to wear a seat belt for health reasons B．not to wear a seat belt for health reasons C．to get a valid medical certificate before wearing a seat belt D．to pay a fine rather than wear a seat belt