单选题 水的三相点附近，其汽化热和熔化热分别为44.82kJ·mol^-1和5.99kJ·mol^-1。则在三相点附近冰的升华热约为    (    )
   A．38.33kJ·mol^-1    B．50.81kJ·mol^-1
   C．-38.33kJ·mol^-1    D．-50.81kJ·mol^-1