What's the biggest obstacle for the railroads heading east from california?

Why might the greeck city states have an advantage over the persians

How are the processes of asexual reproduction and sexual reproduction different? check all that apply.

1. how did manifest destiny spur nation building in the u.s.? 2. how did the idea of “the nation” begin to change? 3. how were the unifications of germany and italy similar? 4. what problems did the austro-hungarian empire have? 5. what domestic issues did england and france have?