The correct answer is A.
A production possibility curve (PPC) is a graph that shows the maximum combinations of two goods that can be produced with a given amount of resources. A straight-line PPC means that the opportunity cost of producing one good in terms of the other is constant. This means that the cost of producing rice in terms of wheat does not change as more rice is produced.
Option B is incorrect because the opportunity cost of producing one good in terms of the other is constant, not increasing. Option C is incorrect because the fixed costs of rice and wheat are not relevant to the opportunity cost of producing one good in terms of the other. Option D is incorrect because the average cost of production of rice and wheat is not relevant to the opportunity cost of producing one good in terms of the other.