When we were kids，my uncle used to take my cousins and me on trips to Racetrack Playa.Scientists know the flat，dry lake bed in California's Death Valley National Park for one strange phenomenon：its slithering stones.Since the late 1940s，they've been studying how sizable stones across the land---some as big as mini fridges--seemingly move thousands of feet on their own.
   There were some seemingly reasonable or absurd theories，but no one had ever seen the rocks in action.Death Valley，as its name implies，is unwelcoming.Put aside that it's one of the hottest places on Earth，but it's also hit by severe winter storms.No one wanted to sit around and watch.
    But in 2011，my cousin Jim，an engineer，came to me with an idea.He suggested setting up cameras near the stones and letting the equipment gather data.The National Park Service said we couldn't leave a trace（痕跡），so we drilled into our own stores，fixed GPS trackers，and placed them，along with cameras，near the rocks.For years，nothing moved.Then in 2013，we got lucky.
   We'd driven eight hours to the park to change the batteries in the GPS，just as we did every few months.This time，though，we discovered there'd been a huge snowstorm.The lake bed was full of water and it was freezing fjye.ait.As we walked around in the moonlight，the whole lake was hissing（嘶嘶）--the sound of ice freezing.But by the next morning，the sun was shinning，and everything was melting.As the thin sheets of ice started to move quickly across the melting water，to our astonishment，they pushed the rocks with them.
   It was a historic moment.Previous theories had gotten close，but we were the first people to directly observe it.And no wonder：It all happened so fast.If you got too involved in your lunch，it'd be over.Fortunately，we were quick to notice，and our GPS trackers recorded a ton of high-quality supporting evidence.

（1）What does the underlined word "slithering" in paragraph 1 mean？

A

A

A．sailing
B．decreasing
C．expanding
D．disappearing
（2）Why Racetrack Playa is called Death Valley？

D

D

A．Because many people died there.
B．Because it used to be a battlefield.
C．Because it is not open to the public.
D．Because it is known for its extreme weather.
（3）What was the function of melting ice during this strange scene？

D

D

A．It froze the rocks.
B．It kept the rocks in shape.
C．It melted the rocks.
D．It carried the rocks along.
（4）What can be used to describe the cousins' feeling while witnessing the historic moment？

B

B

A．Union is strength.
B．Hard work pays off.
C．Seeing is believing.
D．It's never too old to learn.