As used in the 3ut paragraph of this passage, the word "documnted" means ______.

A．the beneficial use of microwaves
B．the harmful effects of microwaves
C．the modern methods of cooking
D．the use and possible harmful effects of microwaves

A．the heat created in microwave ovens is very difficult to control
B．food tastes better when cooked in a microwave oven
C．food cooked in microwave ovens is probably more nutritious than food cooked in more conventional ways
D．most research on the effects of microwaves on human beings has been restricted to the study of low level microwave radiation