4) distillation of crude oil from various parts of the world yields different percentages of hydrocarbons. which statement explains these different percentages?

Answer is: The proportions of components in a mixture can vary.

Crude oil is mixture (solution) consist of several different hydrocarbons (alkanes, alkenes...). This hydroalkanes can be separated from mixture (crude oil) using distilation (usually fractional distillation), because they have different boiling points.  

Proportion of hydrocarbons is diiferent in varios parts of the world.

Solution is homogeneous mixture composed of two or more substances.

The proportions of components in a mixture can vary.

Explanation:

answer: the rate of diffusion increases with increase in temperature.

increase in temperature, increases the molecular/particles speed (kinetic energy. diffusion means movement of particles through the material.

generally, the heavier the particle the more it interacts with the environment and more the temperature matters. so higher temperature results in higher diffusion.

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.