Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A):$ During pregnancy,the level of thyroxine is increased in the maternal blood.
Reason $(R):$ Pregnancy is characterised by metabolic changes in the mother.
In the light of the above statements,choose the most appropriate answer from the options given below:

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