help me! Please.THX

A.(1) only.

Explanation:
- Statement (1) mentioned in reference (the flame will burn hotter);
- No statement (1) in D; therefore not D.
- Statement (2) is not mentioned in the reference; therefore not B and not C.

From Wikipedia:
If the collar at the bottom of the tube is adjusted so more air can mix with the gas before combustion, the flame will burn hotter, appearing blue as a result. If the holes are closed, the gas will only mix with ambient air at the point of combustion, that is, only after it has exited the tube at the top. This reduced mixing produces an incomplete reaction, producing a cooler but brighter yellow flame which is often called the "safety flame". The yellow flame is luminous due to small soot particles in the flame which are heated to incandescence. When the burner is regulated to produce a hot, blue flame it can be nearly invisible against some backgrounds. Increasing the amount of fuel gas flow through the tube by opening the needle valve will of course increase the size of the flame. However, unless the airflow is adjusted as well, the flame temperature will decrease because an increased amount of gas is now mixed with the same amount of air, starving the flame of oxygen.

C,可能錯,不過我D英文都有番咁上下水準架~

應該係(c)

correct ans is C.

The answer is (C).