If a paper moistened with lead acetate for five minutes when placed in manhole turns black. The sewer certainly contains

ANS:A - Hydrogen sulphide

If a paper moistened with lead acetate turns black when placed in a manhole, it indicates the presence of hydrogen sulfide gas (H2S). Lead acetate paper is commonly used as a detector for hydrogen sulfide because hydrogen sulfide reacts with lead acetate to form lead sulfide, which is black in color. So, the correct answer is: Hydrogen sulfide.