But-2-ene-1,4-diol is changed in two stages via an intermediate X to give oxobutanedioic acid. HOCH$_2$CH=CHCH$_2$OH $\xrightarrow{\text{step 1}}$ X $\xrightarrow[\text{hot, acidified KMnO}_4]{\text{step 2}}$ HO$_2$CCOCH$_2$CO$_2$H What reagent could be used for step 1, and what is the intermediate X?
- Areagent: cold, acidified KMnO$_4$; X: HOCH$_2$CH$_2$CH(OH)CH$_2$OH
- Breagent: hot, acidified K$_2$Cr$_2$O$_7$; X: HO$_2$CCH=CHCO$_2$H
- Creagent: steam and concentrated H$_2$SO$_4$; X: HOCH$_2$CH(OH)CH$_2$CH$_2$OH
- Dreagent: warm, acidified K$_2$Cr$_2$O$_7$; X: OHCCH(OH)CH$_2$CHO