单选题 .  Before 1945, hardly anyone outside of New Mexico had ever heard of Alamogordo. In 1960 its population numbered 21,723. Ever since 1898, when the town had been built by the Southern Pacific Railroad, Alamogordo had been a lonely town. The land around it was largely desert, and largely empty.
    Because it was isolated and because the weather was almost always clear and beautiful; a spot of desert near Alamogordo was chosen as the test site for the first atomic bomb ever exploded. The secret name of the test was Zero.
    At dawn on July 16, 1945, the atomic bomb was set off. Observers agreed that they had witnessed something unlike anything ever seen by men before: a huge colorful fireball, more brilliant than the sun, flashing as it rose for miles into the air. Never before had men released so much power at one time, nor had any nation ever possessed a weapon as terrible and destructive as the atomic bomb.
    For several weeks, the test was kept secret. When an atomic bomb was dropped from an American plane on Hiroshima, Japan, newspapers and radio stations all over America told of the test of the bomb in New Mexico. Almost everybody was amazed to learn where the bomb had been made and tested; the deserts of the Southwest had hidden the secret well.
    When news of the atomic bomb and its destructiveness was announced, people all over the world wondered what other new weapons were being prepared in New Mexico desert. Some people doubted that the secret making atomic bombs could be kept from other countries. Some even doubted the wisdom of using so powerful a weapon. But no one doubted that a new kind of war—and a new kind of world—had begun at Alamogordo, one summer morning in 1946.1.  What is the main topic of this passage? ______