At what point on the earth do the rays of the sun strike at a 90 degree angle?

Geologists know that potassium-40 decays to argon-40, with a half-life of 1.3 billion years. if an analysis of a feldspar crystal in a sample of granite shows that 75 percent of the potassium-40 atoms have decayed to form argon-40, what is the age of the granite

Adesert is an example of a(n) and the gobi desert is an example of a(n) a. ecosystem biome b. biome ecosystem c. ecosystem climate d. climate biome

What are the answers for the following questions?