$28 \cdot 18$ In an exam, there are 5 questions in total. After the exam, the score statistics are as follows: $81 \%$ of the students got the first question right; $91 \%$ of the students got the second question right; $85 \%$ of the students got the third question right; $79 \%$ of the students got the fourth question right; $74 \%$ of the students got the fifth question right.

If students who got three or more (including three) questions right are considered qualified, then the minimum pass rate for this exam is
(A) $70 \%$.
(B) $74 \%$.
(C) $81 \%$.
(D) $73 \%$.
(“Zu Chongzhi Cup” Junior High School Mathematics Invitational Competition, 1993)
［Solution］Assume the number of examinees is 100, then we have
$$
19+9+15+21+26=90
$$
(people-questions) answered incorrectly.
If each person misses three questions to be considered unqualified, and $90 \div 3=30$, then at most 30 people can be unqualified. On the other hand, it is possible for the 90 incorrect answers to be concentrated among 30 people, each missing exactly 3 questions, for example:
6 people miss questions $1, 4, 5$; 9 people miss questions $1, 2, 5$; 4 people miss questions $1, 3, 4$; 11 people miss questions $3, 4, 5$. In this way, the pass rate can be at least $70\%$.
Therefore, the answer is $(A)$.