② Lack of "iron" in human body can lead to anemia;

③ The starch in purple sweet potato can provide energy for human body;

According to the characteristics of Chinese characters, selenium is a nonmetallic element, so A is wrong; The atomic number of selenium is 34. According to atomic number = proton number, we can know that its proton number is 34, so B is correct; According to the number of protons in the atom = the number of electrons outside the nucleus, we can know that the number of electrons outside the nucleus is 34, and then we can know that x=34-2-8-6= 18, so C is correct; The unit of relative atomic mass is 1, so d is wrong;

(2) The main component of natural gas is methane. When methane and oxygen are ignited, carbon dioxide and water will be produced. The equation is: CH4+2O2? Light it? . ? CO2+2H2O；

(3) (1) Hydrogen is the most ideal fuel, which has the characteristics of more heat release, wide sources and no pollution.

② Carbon dioxide is tested with clear limewater, and carbon dioxide reacts with calcium hydroxide to generate calcium carbonate precipitate and water. The equation is: Ca (OH) 2+CO2 = CaCO3 ↓+H2O;

So the answer is:

( 1)①C； ? ② Anemia; ? ③ starch; ? ④ BC;

(2)CH4+2O2？ Light it? . ? CO2+2H2O；

(3)① hydrogen; ? ②Ca(OH)2+CO2=CaCO3↓+H2O