A school bus has 25 seats, with 5 rows of 5 seats. 15 students from the first grade and 5 students from the second grade travel in the bus. How many ways can the students be seated if all of the second-grade students occupy the first row?

25P20
5P5 × 20P15
5C5 × 25C14
5P5 × 15P15
5P5 × 25C15