一台电－气阀门定位器的回差过大，产生的原因是（）。A、力矩马达工作不良 B、某一固定

各梁腹板腐蚀测量部位：以（）为准。

A、15%处

B、30%处

C、50%处

D、腐蚀最深处

喷浆过程中根据（）决定开水量大小。

A.链带速度

B.铸铁块度

C.铁块型号

领导决策的基础和依据是( )。

A．信息资源
B．领导的经验
C．领导的判断力
D．单位硬件资源

地西泮的镇静催眠作用机制是______。

A．作用于DA受体
B．作用于受体、
C．作用于5-HT受体
D．作用于M受体
E．作用于α2受体

The sun today is a yellow dwarf star. It is fueled by thermonuclear reactions near its center that convert hydrogen to helium. The sun has existed in its present state for about 4 billion, 600 million years and is thousands of times larger than the Earth.
By studying other stars, astronomers can predict what the rest of the sun’s life will be like. About 5 billion years from now, the core of the sun will shrink and become hotter. The surface temperature will fail. The higher temperature of the center will increase the rate of thermonuclear reactions. The outer regions of the sun will expand approximately 35 million miles, about the distance to mercury, which is the closest planet to the sun. The sun will then be a red giant star. Temperatures on the earth will become too hot for life to exist.
Once the sun has used up its thermonuclear energy as a red giant, it will begin to shrink. After it shrinks to the size of the earth, it will become a white dwarf star. The sun may throw off huge amounts of gases in violent eruptions called nova explosions as it changes from a red giant to a white dwarf.
After billions of years as a white dwarf, the Sun will have used up all its fuel and will have lost its heat. Such a star is called a black dwarf. After the Sun has become a black dwarf, the earth will be dark and cold. If any atmosphere remains there, it will have frozen onto the earth’s surface.

What will probably be the first stage of change as the sun becomes a red giant

A．Its core will cool off and use less fuel.
B．Its surface will become hotter and shrink.
C．It will use up its thermonuclear energy.
D．Its center will grow smaller and hotter.