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Determine whether the conclusion is valid. Tell if it's biased or unbiased as well, and name the sampling method.
A random sample of students at a middle school shows that 10 students prefer listening to rock, 15 students prefer listening to hip hop, and 25 students prefer no music while they exercise. It can be concluded that half of the students prefer no music while they exercise.
What is valid; unbiased; simple random sample?