1) In what ways is the heart unlike most human-built pumps? 2) Why do we need a circulatory system anyway? 3) What structures ensure unidirectional flow?

1.) The heart is unlike many man made pumps because it's more like multiple pumps. The heart pumps blood throughout the whole body and to most organs like the lungs. Man made pumps could never go as fast and filter out the blood to our small circulatory and organ systems.  

2.) We need a circulatory system because it carries blood throughout our bodies. Without it we wouldn't get oxygen to our lungs or heart therefore they'd stop working.

3.) Some structures that ensure unidirectional flow are Heart Valves which ensure that unidirectional blood flow reaches through the cardiac cycle. Some examples of these valves are, atrioventricular valves, tricuspid valves, and the mitral valves.

Hope this helps. Was a week late but it showed up on my recommended. :)

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.