为什么输入21与39也显示是质数

Question

a = int(input('请输入：\n'))
for b in range(2, a-2):
    if a % b == 0:
        print('不是质数')
    elif a % b != 0:
        b += 1
        print('质数')
    break

Answer 1

a = int(input('请输入：\n'))
for b in range(2, a):
    if a % b == 0:
        print('不是质数')
        break
else:
    print('是质数')