题目

原子核是                                                             （    ）  A．由电子和质子构成             B．由质子和中子构成  C．由电子和中子构成           D．由质子、中子、和电子构成 答案：B of the land in that town covered with trees and grass. A .Two fifth; is B. Two fifth; are C. Two fifths; is D. Two fifths; are