The value of van der Waals constant `a` for the gases `O_(2)`, `N_(2)`, `NH_(3)`, and `CH_(4)` are `1.360`, `1.390`, `4.170`, and `2.253 L^(2) atm mol^(-2)`, respectively. The gas which can most easily be liquefied is
A. `O_(2)`
B. `N_(2)`
C. `NH_(3)`
D. `CH_(4)`