Why does opportunity cost increases along with the ppf?

Opportunity cost increases along the production possibilities frontier (PPF) because resources are not perfectly adaptable for the production of different goods. As production shifts from one good to another, increasingly less efficient resources are utilized, leading to a higher opportunity cost for each additional unit produced. This reflects the principle of diminishing returns, where reallocating resources results in progressively larger sacrifices of one good for another. Thus, the slope of the PPF becomes steeper as one moves along the curve.