A：钙单质
B：钙原子
C：钙元素
D：钙离子
The headmaster is so wise that we believe he will lead the school to _____ future.
—Peter, please ask Tom to my office as soon as he _______ back tomorrow.
—OK, I will.
石墨烯又称单层墨。最近美国研究人员通过引入由多层石墨烯制成的交替散热通道，解决了在交通信号灯和电动汽车中使用半导体材料散热的难题。这利用的是石墨烯的（ ）
