下列关于遗传变异和进化的叙述，正确的是 ①双链DNA分子都具有双螺旋结构 ②tR

Scientists think that there has been life for__31_ of years. However we haven’t found life on other planets _32_ . The earth is a planet and it _33__ around the sun. Seven other planets also go _34_ the sun. The sun and its planets are __35  the solar system. The solar system is a small part of our galaxy.
Scientists __36_ lots of spacecraft to look at other planets in the solar system, and some spacecraft have gone __37_ the solar system. However, no one __38_ any life in space yet.
But why has no one from other planets sent us a message ? Have they tried to send _39_ to us ? _40_so many stars in the universe, are we alone, or is there life out there in space ? We don’t know.

小题1:	A．thousands B．millions C．hundreds D．billion

小题2:	A．already B．never C．yet D．just

小题3:	A．go B．turn C．travel D．goes

小题4:	A．to B．back C．around D．into

小题5:	A．called B．call C．calling D．calls

小题6:	A．send B．sent C．have sent D．will send

小题7:	A．out B．beyond C．into D．up

小题8:	A．discovered B．discovers C．has discovered D．have discovered

小题9:	A．message B．informations C．information D．news

小题10:

A．With B．Have C．Use D．Because