4. The cost of computers is continuously decreasing. If the price of a computer decreases by $\frac{1}{3}$ every 3 years, and the current price of a computer is 8100 yuan, then the price after 9 years will be ( ).
A. 2400 yuan
B. 900 yuan
C. 300 yuan
D. 3600 yuan
4. A. $8100\left(1-\frac{1}{3}\right)^{3}=2400$.