The beginning of 1917 was a very turbulent period during the follow of events of the WWI. Germany announced in January, it would engage against any Allied shipping using its submarine force. In February of that same year the president Wilson told the Congress that the US had broken diplomatic relations with the German government.
In march of that year Germany sank five US vessels, contributing to the already unstable American society. The british propaganda in the US and the deep economical involvement with the Allies carried the country to finally enter the War.
The correct answer is:
An understanding of the number and size of leftover planetesimals in the early solar system.
The giant-impact hypothesis proposes that the Moon was created out of what was left of a collision between Earth and an astronomical body the size of Mars, around 4.5 billion years ago, in the Hadean eon. The heavy bombardment was a period when leftover planetesimals hit planets during the solar system's first hundred million years.
