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.

Why is the huronian supergroup rock formation particularly interesting to scientists? because it looks like a cross-section of a giant, stone encyclopedia because it formed during the period when oxygen began to accumulate in the atmosphere because it contains unusually large amounts of oxygen and sulfur because it dramatically altered the planet's development when it first formed