Health is the eternal theme of human society.

① When people sweat, some inorganic salts will be discharged with sweat. Drinking a cup of light salt solution can supplement the discharged salt, which is beneficial to the body, so it is correct.

Carbon is a constant element needed by human body, so it is wrong.

(3) Calcium is the main component of bones and teeth, which provides them with hard structural support. When lacking, children and adolescents suffer from rickets, and the elderly suffer from osteoporosis; However, too much calcium intake is not good for the body and is prone to cataracts; So the statement is correct.

④ The commercial hematopoiesis agent of Red Heart K is to supplement the iron needed by human body, so it is wrong.

So choose D.