Which row gives a pair of molecules in which the $M_r$ of compound X is exactly half the $M_r$ of compound Y? [$A_r$: C, $12$; Cl, $35.5$; H, $1$; O, $16$]
- A$X: \text{CH}_3\text{CH}_2\text{CH}_3,\; Y: \text{CO}_2$
- B$X: \text{CH}_3\text{CH}_2\text{Cl},\; Y: \text{CH}_2\text{ClCH}_2\text{Cl}$
- C$X: \text{CH}_3\text{CO}_2\text{H},\; Y: \text{CH}_3\text{CH}_2\text{CH}_3$
- D$X: \text{CO}_2,\; Y: \text{CH}_3\text{CO}_2\text{C}_2\text{H}_5$