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13. A grocery store sells 12 bottles of water for $8 and 36 bottles of water for $20.
Is the cost of the water proportional to the number sold? Explain.
a) Proportional because the unit rates are $1.50 per bottle.
b) Proportional because the unit rates are $1.80 per bottle.
c) Nonproportional because the bottles have different unit rates.
d) Nonproportional because one grocery store has more bottles.