我国宪法的民主原则有（）
A、一切权力属于人民
B、民主集中制
C、权利义务致性
D、民族区域自治
参考答案：A, B, C
计算：83÷9≈
Many years of hard work_______ his great success in the development of this new type of material.
A．due to
B．thanks to
C．contributed to
D．looked into
