A bus leaves a bus station at $11:53$.  It travels at a constant speed of $60$km per hour.  How many km has the bus travelled by $12:38$? 
选项为:A:$80\rm km$  B:$30\rm km$  C:$60\rm km$  D:$75\rm km$  E:$45\rm km$  请回答其中一个选项.