The dates assigned to earth and to its prehistoric remains are:

forever constant
always predictable
only estimates
accurate within a few years

the dates assigned to the Earth are only estimates, nobody knows the EXACT date of the Earth

a. because the u.s. was the leading with the most technology, economy, etc. also was teamed with other allies witch boosted the economy and materials to improve technology and use it.

) the chinese armed forces had been removed from the city