Assertion: In isochoric process $p-V$ graph is straight line parallel to $p-$ axis. 
Reason : In isochoric process density $p$ remains constant.

If (A) is incorrect but (R) is correct 

If both (A) and (R) are correct but (R) is not the correct explanation of (A)

If both (A) and (R) are correct and (R) is the correct explanation of the (A)

If (A) is correct but (R) is incorrect