Assertion. Prothrombin is essential for blood clotting .
Reason Prothrombin is synthesized in the liver in the presence of `Ca^(++)`.
A. If both A and R are true and R is the correct explantion of A.
B. If both A and R are true but R is not the correct explanation of A.
C. If A is true but R is false
D. If both A and R are false.