题干
China's high-speed trains _________ and perfected now.
A：developed
B：have developed
C：are being developed
D：will have been developed
答案(点此获取答案解析）
C
China's high-speed trains _________ and perfected now.
A：developed
B：have developed
C：are being developed
D：will have been developed
，则对于热化学方程式：
，下列说法正确的是（ ）
