The answer C looks the best to me. It states the fact that it allowed them to practically DOUBLE their land, and it lead to much westward expansion, which later lead to the gold rush, and other important things to America.
Sponsorship means making key investment decisions and providing top-level endorsement of the rationale and objectives of a programme or project. It also means continuing senior level commitment and support for proposed change and championing the implementation of the new capabilities being delivered.
