Which molecules of the adaptive defense system provide humoral immunity by circulating freely in the blood and lymph, where they bind to extracellular antigens and inactivate them and mark them for destruction?

Adaptive immune defense system consists of lymphocytes like B-lymphocytes and T- lymphocytes. B-lymphocytes provides humoral immunity while T- lymphocytes provide cell-mediated immunity to the body.

99% of lymphocytes circulate freely in the blood and lymph. B lymphocytes differentiate into plasma B cells and B memory cells when interact with antigen presented by T helper cells.  

Then plasma cells secrete antibodies in the circulation which binds to extracellular antigens through antigen-binding site. Then the bounded antigen is recognized by receptors present on phagocytic cells. This receptor binds the Fc region of antigen bounded antibody and destroy the antigen by phagocytosis.

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.