1971March 15, the State Council, USA, announced the lifting of all restrictions on using American passports to travel to China. On April 14, Nixon announced the end of the existing.

Analysis of test questions: This question mainly examines Sino-US relations. Pay attention to the understanding of the material when answering this question. The cancellation of "restriction and prohibition" in the materials shows that the United States has the possibility of reconciliation with China, which paved the way for Nixon's visit to China later, so A is correct; B mistake, after the reform and opening up; C the wrong statement is not a historical fact; D was signed on 1972 when Nixon visited China. So you should choose a.