What development has led many world leaders to fear that a nuclear arms race had started

The US government's decision to develop a hydrogen bomb, first tested in 1952, committed the United States to an ever-escalating arms race with the Soviet Union. The arms race led many Americans to fear that nuclear war could happen at any time, and the US government urged citizens to prepare to survive an atomic bomb. Their reason for this panicked prognosis was the United States' decision to develop and test a hydrogen bomb, or H-bomb, a nuclear weapon one thousand times more powerful than the atomic bomb that had leveled Hiroshima at the end of World War II.

Explanation:

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain

valentina tereshkova

soviet cosmonaut valentina tereshkova became the first woman to fly to space when she launched on the vostok 6 mission june 16, 1963. valentina tereshkova was the first woman to go into space. in 1963, she spent almost three days in space and orbited earth 48 times in her space capsule, vostok 6.jan 22, 2018