A:冶铁技术的产生
B:犁铧的使用
C:犁壁的使用
D:曲辕犁的创制
68-50=____
48-30=____
38-20=____
28-10=____
中国终于有了自制的航空母舰,这确实值得骄傲。
China has owned its first homemade aircraft carrier finally. That's really something to be ____ ____.