听力原文: The FBI warned a planned terror attack may occur in the United States or against
U. S. interests in Yemen, as reported on Tuesday, but U. S. officials do not know the target or how the suspected plot would be carried out. The FBI said the uncorroborated threat came from information discovered in Afghanistan, where the U. S. military has been sifting through material left by al Qaeda members, and from interviews with detainees, including al Qaeda members, held at the U. S. Navy Base at Guantanamo Bay, Cuba. Rushing to put out the alert late Monday night after recently discovering the possible threat, the FBI said one or more operatives might be involved in the attack, including a Saudi-born Yemeni man.
The FBI sounded a warning against a possible attack in the United States because
A.they don't know how the plot will be carried out.
B.they don't know what target the plot aims at.
C.they have got information from some material.
D.they have sifted through material for information.