Responding to xenophobic concerns, congress passed an emergency law restricting immigration in 1921. among other provisions, the act established ( a proportional share of a total) for nationalities on the basis of their numbers in the u. s. in 1910

Responding to xenophobic concerns, Congress passed an emergency law restricting immigration in 1921. Among other provisions, the Act established a quota (a proportional share of a total) for nationalities on the basis of their numbers in the U.S. in 1910.

The missing detail is: a quota.