50x you must take the magnification of the ocular lens (5x) and multiply it by the magnification of the low power lens (10x).
5X10=50
A baby's heart beat 60-90 per minute an athlet beat 40-46
Well...I need choices but, if these are one of the choices i'll go with A is conceptually the same as "low power" and B is conceptually the same as "high power".
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I think it might be D Circulatory