Assertion(A) : The enthalpy of formation of `O_(2)` at `1` atmospheric pressure and `298K` is zero.
Reason(R ) : The enthalpy of formation of all the elements at STP is zero.
A. Both `A`and `R` are true and `R` is the correct explantion of `A`
B. Both `A`and `R` are true and `R` is not a correct explantion of `A`
C. `A` is true but `R` is false
D. `A` is false but `R` is true
