17. The price of a TV set was reduced by $10 \%$ twice in succession, and the price after the reduction is $a$ yuan per set. Then the original price of the TV set is ( ).
(A) $0.81 a$ yuan
(B) $1.21 a$ yuan
(C) $\frac{a}{1.21}$ yuan
(D) $\frac{a}{0.81}$ yuan
Reference answer: D
