In natural numbers, every multiple of 5 () a must be prime, B must be composite, and the first two terms may be urgent

In natural numbers, every multiple of 5 () a must be prime, B must be composite, and the first two terms may be urgent

Choose C

Among the natural numbers from 1 to 10, the number of multiples of 3 accounts for (), the number of multiples of 5 accounts for (), the prime number accounts for (), and the total number accounts for ()

Among the natural numbers from 1 to 10, the number of multiples of 3 accounts for (30)%, the number of multiples of 5 accounts for (20)%, the prime number accounts for (40)%, and the total number accounts for (50)%
The multiples of 3 are 3, 6 and 9
The multiples of 5 are 5 and 10
Prime numbers are 2, 3, 5 and 7
The total number is 4, 6, 8, 9 and 10
Note that 1 is neither prime nor composite

The prime number has () and the combined number has (); 1,27,37,41,51,57,69,83,87

Prime numbers have (37,41,83) and combined numbers have (27,51,57,69,87)

The integral part of 1 / (1 / 1980 + 1 / 1981 + 1 / 1982 +. 1 / 1991)

Let s = 1 / 1980 + 1 / 1981 + 1 / 1982 +. 1 / 1991, then the original formula = 1 / S; in fractions, if the numerator is the same, the greater the denominator, the smaller the denominator
①S=1/1980 + 1/1981 + 1/1982 +...+1/1991>1/1991 + 1/1911 + 1/1991 +...+1/1991=12/1991
②S=1/1980 + 1/1981 + 1/1982 +...+1/1991