Đáp án:

 A

Giải thích các bước giải:

Ta có: 

\(\begin{array}{l}
f = \dfrac{\omega }{{2\pi }} = \dfrac{1}{{2\pi \sqrt {LC} }}\\
 \Rightarrow \dfrac{{{f_1}}}{{{f_2}}} = \sqrt {\dfrac{{{C_2}}}{{{C_1}}}}  = \dfrac{{30}}{{40}} = \dfrac{3}{4}\\
 \Rightarrow \dfrac{{{C_2}}}{{{C_1}}} = \dfrac{9}{{16}} \Rightarrow {C_2} = \dfrac{9}{{16}}{C_1}\\
C = \dfrac{{{C_1}{C_2}}}{{{C_1} + {C_2}}} = \dfrac{{{C_1}.\dfrac{9}{{16}}{C_1}}}{{{C_1} + \dfrac{9}{{16}}{C_1}}} = \dfrac{9}{{25}}{C_1}\\
\dfrac{f}{{{f_1}}} = \sqrt {\dfrac{{{C_1}}}{C}}  = \sqrt {\dfrac{{{C_1}}}{{\dfrac{9}{{25}}{C_1}}}}  = \dfrac{5}{3}\\
 \Rightarrow f = \dfrac{5}{3}{f_1} = \dfrac{5}{3}.30 = 50kHz
\end{array}\)