CR:
1、A
Factors→the economic development of the United States in the 19th century
A: the expansion of the railroad network
B: railroad, urban roadways and highways
2、评价题E
A:the cost of including tourists是指全部成本,无关;C???
RC: ACACC
P1
Saturn's moon卫星 Enceladus → perplex scientist
E stand out→ geological reshaping→active geologically
P2
E: prime candidate for harboring life
P3
E internal heat
Radioactive decay within rocks
Tidal heating ——most credible