The correct answer to this open question is the following.
Although you forgot to attach de options for this question we can comment on the following.
One strategy that the United States is using in an attempt to keep terrorists from entering the country is the following: "It has significantly increased the number of checkpoints along the Mexican and Canadian borders."
Since the terrorist attacks of September 11, 2001, to New York City and other parts of the United States, President George W. Bush ordered the creation of some legislation to protect the United States from the terrorist threat and created the Home Land Security Department.
Furthermore, US Customs significantly increased the number of checkpoints along the Mexican and Canadian borders to better control the people who enter the United States territory.