单项选择题
A．It is secreted by the kidneys into the bloodstream. B．It is hormone that prepares the body for emergencies. C．It is the only useful hormone produced in the body. D．It is essential to sustain lif
A．has a great deal of trouble doing mental arithmetic B．lacks some of the qualities required of a scientist C．just reads about other people’s observations and discoveries D．comes up with solutions in a most natural way
A．laser-doppler radar is more accurate than microwave radar B．microwave radar may not ensure absolute precision in measuring varying speeds C．the light-beam radar can measure only slow speeds with accuracy D．microwave radar operates at a higher frequency than laser-doppler radar