The number of students sampled is: 105%=200 (people),

80 ≤ x < 90: 200×30%=60 (people),

The percentage of 90 ≤ x < 100 is: 50200× 100%=25%,

The frequency of 70 ≤ x < 80 is: 200× (1-5%-10%-30%-25%) = 60 (person),

Then the number of seventh-grade students who won the second prize is about 60+60200×2000= 1200 (person);

So choose a.