Which of the following is true of clouds at medium altitudes?

a. they are a sign of severe weather.
b. they generally produce lightning.
c. they are made of ice crystals.
d. they cause light rain and snow.

The one that is true of clouds at medium altitudes is : 
B. they generally produce lightning
It happens because of the movement of the electrons in the cloud that triggers a spark of electricity

Intrusion is the best answer and the newest form.

Evidence peice 1: millions of years ago existed the supercontinent pangea after millions of yearsnit broke into peices and formed smaller continents and islandsevidence peice 2: there is a massive fault line in san fransico and california that has the potential to cause massive earthquakes which supports the theory of fault linesevidence peice 3: mountains are said to be formed by earths tectonic plates crashing against each other for millions of years and eventually if forms a big point know as a mountain.evidence peice 4: the mariana trech is the deepest place (so far) discovered in the oceans of earth. it is so deep that you can literally see how earths plates are divided and or continents as well