问题：

[单选,材料题] 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 of snow．Huge snow slides are most common in mountains where there are steep slopes that are well buried in snow and ice．The snow builds up slowly and lands very softly．This can create a very touchy, unstable situation．Tons of snow may be held up by only the friction between snowflakes．The deep snow is like a house of cards．The slightest movement can cause it to fall．As soon as something slips, this great mass of snow will come crashing down the mountainside．Slides may be started by sound vibrations．They may also be started by the weight of wet, melting snow．Once an avalanche has been triggered, the cause no longer matters．Moving down a steep slope, it picks up great speed and added snow．Some avalanches travel as fast as two hundred miles per hour．The force of an avalanche will mow down anything in its path．Whole houses have been swallowed up by these fast-paced piles of snow．The wind that is caused by an avalanche is almost as destructive as the snow itself．Winds from an avalanche have been known to travel as fast as those of a tornado．So, when approaching a thickly snow covered mountain, speak softly!

Avalanches can be started by________

A . frightening
B . fast
C . shaky
D . undesirable