Explanation:
Advertisement cost per thousand copies in 1997:
\(\text{Cost} = \frac{5000}{25000} \times 1000 = 200\)
Advertisement cost per thousand copies in 1998:
\(\text{Cost} = \frac{20000}{30000} \times 1000 = 666.67\)
Comparison:
Since the cost in 1998 (666.67) is greater than in 1997 (200), there is an increase.
Increase in cost:
\(\text{Increase} = 666.67 - 200 = 466.67\)
Thus, the advertisement cost increased by Rs. 466.
Therefore, the correct answer is option C.