Zack works at a carwash and can either do interior carpet cleaning, which takes 30 minutes per job, or exterior car washes, which takes 20 minutes per job. He works a maximum of six hours per day. If he is paid $5 for each exterior car wash and $6 for each interior carpet cleaning job, which of the following is true about the number of hours Zack can clean carpets (c) and do exterior car washes (e).

1. The system {c=>0, e=>0, c+e<=6 correctly represents the situation.

2. The critical points (c, e) are (0, 0), (12, 0), and (0, 18).

3. Zack can maximize his earnings by doing 12 carpet cleaning jobs and zero exterior car washes.

4. Zack can maximize his earnings by doing zero carpet cleaning jobs and 12 exterior car washes.