According to the source, what is the profit-maximizing rule for a perfectly competitive firm, stated in terms of marginal revenue and marginal cost?

1. Profit keeps rising as long as an additional unit adds more revenue than it costs, which is MR greater than MC. 2. Profit keeps falling once an additional unit costs more than it adds in revenue, which is MC greater than MR. 3. The turning point between these two zones, and so the profit-maximizing output, is where MR equals MC. 4. Producing the maximum physically possible quantity ignores cost altogether and is not the rule . _Source: OpenStax Principles of Microeconomics for AP Courses (CC BY 4.0), Ch 8 "Perfect Competition", section 8.2 | How Perfectly Competitive Firms Make Output Decisions_