The most significant obstacle in building the transcontinental railroad was

Crossing the Sierra Nevada and the Rocky Mountains

Explanation:

The Transcontinental Railroad was a huge project that lasted from 1863 to 1869 and connected the US Rail network with the Pacific Coast Railroads. There were many obstacles but the most significant one was Sierra Nevada since it was very expensive to make the railroad pass through the mountains to reach the Great Basin. Later they discovered a way across the Sierras that would be suitable for the railroad.

a.) are both appointed for life

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