Because if we're simultaneously moving/manipulating two different variables it can be very hard to determine why one variable is changing and why the other is changing. While this can be done with more complicated experimental designs, it usually is the case that scientists will only manipulate one variable and not two different ones.
Answer:
D. cell, tissue, organ, organ system, organism
Explanation:
also before cells there are also (and in order) atoms, molecules, organelles
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