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, and methane and oxygen are ignited to generate carbon dioxide and water. The equation is: CH 4 +2O 2 ignition. 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) CH 4+O 2 is ignited. CO2+2H2O；
(3)① hydrogen; ②Ca(OH) 2 +CO 2 =CaCO 3 ↓+H 2 O