$6 \cdot 74^{16} \cdot 5^{25}$ The number of digits (in decimal notation) of this number is
(A) 31 .
(B) 30 .
(C) 29 .
(D) 28 .
(E) 27 .
(35th American High School Mathematics Examination, 1984)
[Solution] Express $4^{16} 5^{25}$ in scientific notation (exponential notation), we have
$$
4^{16} \cdot 5^{25}=2^{32} \cdot 5^{25}=2^{7} \cdot 10^{25}=128 \cdot 10^{25} \text {. }
$$

This indicates that there are 25 zeros following 128, so there are a total of 28 digits. Therefore, the answer is $(D)$.