A bag of flour weighs $1.5\rm kg$. Bud uses $300\rm g$ of flour to make a strawberry cake and a further $150\rm g$ of flour to make biscuits.  What fraction of the bag of flour has she used? 
选项为:A:$\frac37$  B:$\frac13$  C:$\frac{7}{15}$  D:$\frac{3}{10}$  E:$\frac{4}{15}$  请回答其中一个选项.