答案：B

解析：

The sum of the positive odd integers less than 50 is 1 + 3 + 5 + ... + 45 + 47 + 49, and the sum of the positive even integers less than or equal to 50 is 2 + 4 + 6 + ... + 46 + 48 + 50. When the first sum is subtracted from the second, the terms can be regrouped to form (2-1) plus (4-3) + ... + (48-47) + (50-49), which is just 1 + 1 + 1 + ... + 1, where there are 25 ones. Therefore, the correct answer is 25.