For which of the following reactions does the enthalpy change for the reaction equal to the standard enthalpy of formation for the product in the reaction; that is, which has $\Delta H_{\text{reaction}} = \Delta H_{\text{f}}^{\circ}$?
(Select all that apply.)
a. $2\text{H}(g) \rightarrow \text{H}_2(g)$
b. $\text{H}_2(g) + \text{CrO}_4(l) \rightarrow \text{H}_2\text{CrO}_4(l)$
c. $2\text{C}(\text{graphite}) + \frac{1}{2}\text{O}_2(g) + 3\text{H}_2(g) \rightarrow \text{C}_2\text{H}_5\text{OH}(l)$
d. $\text{Mn}^{2+}(aq) + 2\text{Cl}^{-}(aq) \rightarrow \text{MnCl}_2(s)$
e. $\text{O}_2(g) \rightarrow \text{O}_2(g)$