Assertion $(A)$: The zeroth law of thermodynamics leads to the concept of temperature.
Reason $(R)$: The zeroth law states that two systems in thermal equilibrium with a third system are in thermal equilibrium with each other.
The correct option among the following is:

• A
  $(A)$ is true, $(R)$ is true and $(R)$ is the correct explanation for $(A)$
• B
  $(A)$ is true, $(R)$ is true but $(R)$ is not the correct explanation for $(A)$
• C
  $(A)$ is true but $(R)$ is false
• D
  $(A)$ is false but $(R)$ is true