What was the most expensive pigment in medieval Europe？ A: Cobalt blue B: Purplish C: Yellow D: Brown

Which of the following is the most abundant solid mineral in the seabed?( ) A: Manganese nodule B: Nickel ore C: Copper mine D: Cobalt ore

As we know,nuclear energy was usedin the USand__________ were developed, two of them were used against Nagasaki and Hiroshima in Japan in August 1945 before the end of World War II. A: the hydrogen bomb B: the medium bullets C: the atom bomb D: the cobalt nuclear bomb