Assertion (A): According to gas equation,

$\dfrac{\text{P}$1\text{V}1}{\text{T}1} = $\dfrac{\text{P}2\text{V}$2}{\text{T}2}

Reason (R): On combining Boyle's law and Charle's law, 'volume of a given mass of a dry gas varies inversely as the pressure and directly as the absolute temperature."

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Assertion (A): The absolute scale of temperature is the Kelvin scale.

Reason (R): Kelvin scale starts from 0°C.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Assertion (A): One litre of hydrogen weighs the same as one litre of oxygen.

Reason (R): One litre of hydrogen contains the same number of molecules as one litre of oxygen.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Assertion (A): Atomic mass is expressed in atomic mass unit.

Reason (R): Atomic mass is defined as $\dfrac{1}{12}$ times the mass of carbon-12.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.