A、Always. B、Once. C、Twice. D、Three times. D在原文中听到"Their research found that there were only three times in Earth’s history when

Diamonds are the hardest substance known on Earth and they make sparkling jewelry as well as sawblades that can cut through pretty much anything. They form deep within the Earth’s mantle layer ( about 200 km deep ) when atoms of the element carbon arrange themselves into a lattice under huge amounts of pressure from the load of rocks above.
    Recently, geologists studying diamonds have identified the ages of thousands of diamonds from Southern Africa, where the mineral is most abundant. Their research found that there were only three times in Earth’s history when diamonds were made and that the Earth no longer makes diamonds like it used to. Something was different then. Perhaps the planet was hotter on the inside, or the composition of the rocks was subtly different. "Whatever it was it has changed now," stated Steve Shirey of the Carnegie Institution in Washington, D.C., one of the project researchers.
    The oldest diamonds were made 3.3 billion years ago when Earth was relatively young. The second time diamonds formed was 2.9 billion years ago. The youngest diamonds on Earth are 1.2 billion years old although a few smaller diamonds are about 100 million years old. Small bits of rock within those diamonds, called impurities, indicate that they probably formed from rocks laid down in a shallow sea suggesting that the carbon that made the diamonds probably came from the remains of ancient sea life.
    Some people like diamonds because they are pretty, rare, or expensive but Dr. Steve Shiley has a different perspective. "I think of diamonds," he said, "as being tiny time capsules that encase a little piece of rock protecting it for billions of years and providing us with a unique window on ancient times."