Assertion $(A)$: When a spring is cut into two equal parts,the spring constant of each piece is twice that of the original spring. Reason $(R)$: Spring constant is inversely proportional to the length of the spring.

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