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Consider Darrells Engel curve below for pairs of shoes. Note that at $0$ income, Darrell buys $0$ pairs of shoes.
If Darrell's income doubles, how will the amount of shoes he purchases change?
Darrell buys two more pairs of shoes.
The amount of shoes that Darrell buys increases by 50\%.
Darrell buys half as many shoes.
Darrell buys twice as many shoes.
There is not enough information to tell.