B, check whether the oxygen collection is sufficient, put the wood with Mars in the bottle mouth, not in the bottle. The operation shown in the figure is wrong.
C. For the operation of dropping a small amount of liquid with a rubber-tipped dropper, pay attention that the rubber-tipped dropper cannot extend into the test tube or touch the inner wall of the test tube, and drop the liquid vertically above the mouth of the test tube to prevent the rubber-tipped dropper from being polluted. The operation shown in the figure is wrong.
D. When smelling gas, gently fan the bottle mouth with your hands to make a very small amount of gas float into your nose. You can't put your nose on the mouth of the gas container to smell the gas. The operation shown in the figure is wrong.
So choose: a.