Tropical ocean waters are more saline than colder waters because tropical waters

The answer is; are warmer

The amount of solute (this case salts) that a solvent (this case water) can dissolve is directly proportional to the temperatures of the solution. The higher the temperatures the more solute is dissolved before saturation is reached. Tropical waters are warm due to the angle and surface areas at which the water hit the waters, as compared to the polar regions.  

the correct answer is - d. china has a large and growing population.

the fact that china has a large (the biggest) population, and it is still growing, doesn't hinder the agriculture production in the country. in fact, it can for increased production in the agriculture, since there's more labor force, as well the sheer mass of people produces lots of scientists that come up with new inventions as to how to improve this sector. nice example is the inner mongolia region in china, in general dominated by the gobi desert, but few scientist came up with a nice idea as to how to make the region productive by using plants that are very resistant to droughts, so lots of people have been employed, and the region is becoming greener and greener each year, and the production of food in this region is also increasing.