The horse of 50 million years ago, called the Dawn Horse, was a little animal the size of a fox terrier. The species had four toes on each front foot and three toes on each hind foot. Its “toenails” were little hooves. When this creature lived, there were no grasslands. Its home was the forest, __19__ it fed on tender shoots and leaves.

       There is proof that this little creature was __20__ of the horse of today. Scientists have excavated fossils of certain animals that lived a few million years later and found that, __21__ they were bigger than the animal of earlier times, they resembled it and the modern-day horse.

       The horse family survived when many other animals died out __22__ it had two advantages. The little horse was swift, as we can guess from its slim body and slender legs. It also was fairly intelligent； its skull shows that its brain was large __23__ its body.

       Uranus is perhaps the strangest planet in the solar system. At some point during its history, the ice giant was knocked over, leaving it spinning on its side. More than a dozen rings circle the world, and some 27 moons cling to it. The planet’s atmosphere is a collection of hydrogen, helium, and heavier compounds that exist as ices deep in the frigid Uranian clouds. 

      But beyond a handful of intriguing facts, scientists know woefully little about this milky blue world, which was visited for the first and only time by the Voyager 2 spacecraft in 1986. That could be about to change. 

      A report prepared by planetary scientists and released this week has recommended that NASA make a mission to Uranus a top priority for the coming decade, perhaps launching a spacecraft to the seventh planet from the sun—as well as a thousand inevitable jokes—as soon as 2031. 

     One big draw is that Uranus—and Neptune, the solar system’s other ice giant—might be representative of the most common type of planet in the galaxy. Scientists think that solving the mysteries of Uranus, such as its odd magnetic field, shrouded interior structure, and surprisingly frosty temperatures, could be crucial not only for understanding ice giants across the Milky Way, but also for unlocking clues about the history of our solar system.