Lexie lives at sea level, but her family rented a vacation cabin high in the Rocky Mountains for two weeks. She noticed a difference in her breathing. An image of a mountain with three circles going up the side of it. The lowest circle is filled with many, smaller circles. The middle circle has fewer circles that are farther apart. The highest circle has the fewest circles that are the farthest apart.

What do the molecules in the circles model regarding altitude?
A. Air pressure increases at higher altitudes.
B. Air density decreases at higher altitudes.
C. There is a greater gravitational pull on air molecules at higher altitudes.
D. Temperature has a decreasing effect on air molecules at higher altitudes.

B. Air density decreases at higher altitudes

Explanation:

Since air is compressible, air from higher altitudes will compress air from lower altitudes. As a result, air density at lower altitudes will be greater than air density at higher altitudes. Air particles at the highest altitudes, not being acted upon by air above it, will be more spread out.  

"do a testcross with a homozygous recessive plant."

it provides an alternative energy pathway for the reaction to proceed.

a catalyst is a substance that speeds up the rate of a chemical reaction but is not consumed during the course of the reaction.

catalysts increase the rate of reaction without being used up. they do this by lowering the activation energy needed. with a catalyst, more collisions result in a reaction, so the rate of reaction increases. different reactions need different catalysts.

a catalyst works by providing an alternative reaction pathway to the reaction product. the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst.