Which gas is a source of natural groundwater pollution ? a. hydrogen b. radon c. carbon d. oxygen

When you imagine how you'll get from your house to the store, you are a. mental projection b. mental map c. piagetian projection d. cognitive load

5. what is a red giant? can our sun become a red giant? why or why not?

Aplane cuts a pyramid as shown in the diagram. what is the shape of the cross section? a. a hexagon b. a pentagon c. a square d. a triangle