Ms. Jones teaches a class in which 70% of the students are boys. She lists the names of all the students in her class on separate pieces of paper. She places all the boys' names in one hat, all the girls' names in another hat. She mixes each of the hats thoroughly, but separately, before reaching in to draw out 7 boys' names and 3 girls' names. Which of the following most precisely describes Ms. Jones's sampling strategy?