题目

阅读理解 Most people know precious gemstones (宝石) by their appearances. An emerald flashes deep green, a ruby seems to hold a red fire inside, and a diamond shines like a star. It's more difficult to tell where the gem was mined, since a diamond from Australia or Arkansas may appear the same to one from the Democratic Republic of the Congo. However, recently, a team of scientists has found a way to identify a gemstone's origin. Beneath the surface of a gemstone, on the tiny level of atoms and molecules（分子）, lie clues (线索) to its origin. At this year's meeting of the Geologi对任意两个非零的平面向量α和β，定义α?β=α•ββ•β，若平面向量a，b满足|a|≥|b|＞0，a与b的夹角θ∈（0，π3），且a?b和b?a都在集合{n2|n∈Z}中，则a•b=（　　）A．12B．-12C．-32D．32