A middle school contains sixth, seventh, and eighth graders. Each grade level contains approximately the same number of students. The principal wants to find the answer to a survey question affecting each student in the school. He asks a sample group "What time do you get up in the morning?" Which of these characteristics define the best sample?

the options are missing, so I looked for a similar question and found these options:

the answer is: an equal number of students chosen at random from each grade level

Step-by-step explanation:

A good sample must be random and representative of all the population. In this case, the population is all the kids that go to middle school. A good sample would need to be taken at random and all the students should have equal opportunity to be chosen, and the three grades must be included in the same proportion.

correct answer:

a + bi

51

step-by-step explanation:

73-22=51 the   mass number minus the amount of protons gives you the amount of neutrons