Many people today like reading stories about UFO.Many people in different countries are studying the UFO.

    There are some different saying for UFO.Among those sayings,the most popular one for UFO is that UFOs are strange creatures(生物) to the earth from other stars.We can call them aliens.

    UFO scientists and many people believed this saying.Some people said that they saw a few UFOs landing on the earth.They seemed to be objects rather like big plates.Some people said that they saw some strange visitors coming out of the UFOs.

    Some people make mistakes when they say they have seen UFOs high in the sky.Perhaps they see a weather balloon or an uncommon plane or the light they see in the sky comes from the ground.Sometimes they see the light from the moon.

    But these are a lot of others we can't understand.It may take many years to find a clear answer and then people will give UFO another name.

过点$\left(3,1\right)$作圆${\left(x-1\right)}^2+y^2=1$的两条切线，切点分别为A，B，则直线AB的方程为（  ）

从能量转换的角度看，一台四冲程内燃机在一个循环中，存在着机械能转化为内能过程的是（  ）