Colby and Sarah sold newspapers this past summer. Colby earned $4.50 for each newspaper he sold. Sarah's earnings were $35 less than Colby's at the end of the summer. The following expression shows Sarah's earnings:

4.50y − 35

In the expression, what does the variable y represent?

Sarah's earnings at the end of the summer
Colby's earnings at the end of the summer
The number of newspapers that Sarah sold
The number of newspapers that Colby sold