在下面列举的各个实例中，哪些情况机械能是守恒的？（   ）

C

    Yangzhou paper-cutting, with a history of 2,000 years, can be dated back to the Sui Dynasty, making Yangzhou one of the places where paper-cutting first became popular.

    In the Sui Dynasty, the people of Yangzhou would cut colourful paper or silks and satins(缎子) to celebrate festivals. It is said that Emperor Yang came to Yangzhou three times. In winter, the flowers and trees in the garden become dry and weak. The Emperor ordered the girl servants to cut fine silks and satins into flowers and leaves and use them to decorate the trees and lakes, copying nicely the looks of spring and summer. Since then, “colorful brocade cutting” (织锦缎) has become a popular art in Yangzhou.

    In the Tang Dynasty, the paper-cutting industry was highly developed and a large amount of high-quality paper was made as presents to the Court(宫廷).

    In the Qing Dynasty, because of the economic development, the people of Yangzhou became interested in wearing fine clothing, especially embroidered clothing(刺绣). The embroidered designs was based on paper-cuts. The most well-known of them was Bao Jun, a paper-cutting master. He won a high reputation(名声) as Magic Scissors(剪刀) for his wonderful cutting skills.

    After the People's Republic of China was started, the Chinese government accepted the importance of paper-cutting like many other arts and skills. In 2007, China Paper-cuts Museum opened to the public in the back garden of the Wangs' Residence(住所), Yangzhou, greatly helping the development of Yangzhou paper-cutting.

甲常说：“我是万能输血者．”乙却说：“我是万能受血者”．请你分析一下，甲、乙的血型分别是（　　）

中科院的全超导的“人造太阳”一一托克马克核聚变试验装置的调试运行成功，使我国在该领域的研究处于世界前列。氘和氚是核聚变的原料(氘、氚原子核内都只有l个质子，但含不同数目的中子)，聚变发生后，氘、氚原子核转变为氦原子核。根据以上叙述，下列说法正确的是(    )