Chọn đáp án sai 3/7 + 8/21 = 17/21

A. $\frac{3}{7}+\frac{8}{21}=\frac{17}{21}$

B. $\frac{4}{5}-\frac{2}{3}=\frac{2}{15}$

C. $\frac{7}{11}x\frac{3}{11}=\frac{21}{11}$

D. $\frac{1}{3}:\frac{1}{4}=\frac{4}{3}$