如下图所示是集成异步二－五－十进制计数器，用它构成8421码十进制计数器时，需使（）。<img src="https://static.zxtiku.com/wangke/imgs/topic/32b22c1c104366b12b90edd1e9a3c8dc.jpg"/> A. <img src="https://static.zxtiku.com/wangke/imgs/topic/697b6584626a357ce31bf599914f941b.jpg"/> B. <img src="https://static.zxtiku.com/wangke/imgs/topic/9d4a0c527e4a3e652d31fcde81ee4270.jpg"/> C. <img src="https://static.zxtiku.com/wangke/imgs/topic/f07f1fc2f27f93507ab2387c1d2f4a45.jpg"/> D. <img src="https://static.zxtiku.com/wangke/imgs/topic/c21f97817812a564a2712d767af62ca5.jpg"/>