1. scientists have discovered that acid rain is caused by
a. acid evaporating into the atmosphere.
b. sulfur introduced into the atmosphere from burning coal.
c. hydrochloric acid in clouds.
d. gas introduced into the atmosphere from animal wastes.

the median of the data is 4.5.

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.