What is one strategy the united states is using to keep terrorists from entering the country?

it has required people entering along the mexican and canadian borders to surrender their phones.
it has prohibited nighttime crossings by closing checkpoints along the mexican and canadian borders.
it has built large walls on the mexican and canadian borders to keep people out of the country.
it has significantly increased the number of checkpoints along the mexican and canadian borders

Explanation:

The United States Border Patrol works 71 traffic checkpoints, including 33 changeless traffic checkpoints, close to the Mexico– United States border.

The main role of these review stations is to hinder unlawful movement and pirating exercises. After the September 11 assaults in 2001, they assumed the extra job of fear mongering deterrence.[citation needed] These checkpoints are situated somewhere in the range of 25 and 75 miles (40 and 121 km) of the Mexico– United States outskirt along major U.S. thruways; close to the southern outskirt of the bordering United States. Their circumstance at inside areas enable them to hinder illicit exercises that may have skirted official outskirt intersections along the wilderness. The checkpoints are isolated among nine Border Patrol segments. There are some of these checkpoints close to the northern outskirt of the adjacent U.S. too, (for example, in the conditions of New York or Maine); inside 100 miles (160 km) of the Canada– U.S. border.

d. it spread christianity. i just took the test and got it right.

this isn't a question and if you're going to list things be kind and use commas.