En espaol:

Que se puede decir de la concavidad de la frontera de posibilidades? Que significa que tenga esa forma para una economa y que tiene que ver con los rendimientos decreicientes? porque tiene esa forma y que tiene que ver con la curva de contrato?

In english:

What can be said about the concavity of the frontier of possibilities? What does it mean to have that shape for an economy, and what does it have to do with diminishing returns? Why does it have that shape and what does it have to do with the contract curve?