Which of the following reactions would result in a decrease in entropy (i.e., $\Delta S<0$) under standard conditions?

• A. $\mathrm{NH}{4}_{}\mathrm{Cl}\left(s\right)\to \mathrm{NH}{3}_{}\left(g\right)+\mathrm{HCl}\left(g\right)$
• B. $2H{2}_{}\left(g\right)+O{2}_{}\left(g\right)\to 2H{2}_{}O\left(l\right)$
• C. $\mathrm{CaCO}{3}_{}\left(s\right)\to \mathrm{CaO}\left(s\right)+\mathrm{CO}{2}_{}\left(g\right)$
• D. $2\mathrm{SO}{2}_{}\left(g\right)+O{2}_{}\left(g\right)\to 2\mathrm{SO}{3}_{}\left(g\right)$

Answer: (B)