题目内容：
In this passage "unstable"means________回答下面的题目Never laugh at a snow covered mountain!Laughter and yelling, during the avalanche season, can cause a deadly pile ofsnow．Huge snow slides are most common in mountains where there are steep slopesthat are well buried in snow and icE．The snow builds up slowly and lands verysoftly．This can create a very touchy, unstable situation．Tons of snowmay be held up by only the friction between snowflakes．The deep snow is like ahouse of cards．The slightest movement can cause it to fall．As soon assomething slips, this great mass of snow will come crashing down themountainsidE．Slides may be started by sound vibrations．They may also be started by the weight of wet, melting snow．Once an avalanchehas been triggered, the cause no longer matters．Moving down a steep slope, itpicks up great speed and added snow．Some avalanches travel as fast as twohundred miles per hour．The force of an avalanche will mow down anything in itspath．Whole houses have been swallowed up by these fast-paced piles of snow．Thewind that is caused by an avalanche is almost as destructive as the snowitselF．Winds from an avalanche have been known to travel as fast as those of atornado．So, when approaching a thicklysnow covered mountain, speak softly!Avalanches can be started by________A．frightening
B．fast
C．shaky
D．undesirable
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