答案是A，看看维基百科上的解释：
One notable use is as the coolant in experimental fast neutron nuclear reactors. Unlike commercial plants, these are frequently shut down and defuelled. Use of lead or pure sodium, the other materials used in practical reactors, would require continual heating to maintain the coolant as a liquid. Use of NaK overcomes this. NaK is used in many other heat transfer applications for similar reasons.
The Soviet RORSAT radar satellites were powered by a NaK-cooled reactor. Apart from the wide liquid temperature range, NaK has a very low vapor pressure, important in the vacuum of space.（相当大的温度范围内仍然是液态，此外，钾钠合金蒸气压非常低，这一点在真空条件下显得尤为重要。）
