The speeds of Selena and Polly is $4:7$. It takes Selena $28$ minutes to complete a race. How long does it take Polly to complete the same race? 
选项为:A:$49\rm\textasciitilde minutes$  B:$7\rm\textasciitilde minutes$  C:$4\rm\textasciitilde minutes$  D:$16\rm\textasciitilde minutes$  请回答其中一个选项.