Which of the following pairs will give displacement reactions ?

Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it, as shown in figure below.

$(a)$ What will be the action of gas on

$(i)$ dry litmus paper ?

$(ii)$ moist litmus paper ?

$(b)$ Write a balanced chemical equation for the reaction taking place.

Name two metals which will displace hydrogen from dilute acids, and two metals which will not.

Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).

State two ways to prevent the rusting of iron.