Chọn câu đúng 19.20/19+20=1/19+1/20 6+23/11=(6.23+11)/11

A. $\frac{19.20}{19+20}=\frac{1}{19}+\frac{1}{20}$

B. $6\frac{23}{11}=\frac{6.23+11}{11}$

C. $a\frac{a}{99}=\frac{100a}{99}\left(a\in N*\right)$

D. $1\frac{15}{23}=\frac{1.23}{15}$