The Museum's aircraft is an M-21, the first of the rare two-seat variants of the early A-12. Built for a CIA program code-named "Tagboard," it carried an unpiloted D-21 drone for intelligence gathering.

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On December 22, 1964, the first flight of the D-21/M-21 combination (now referred to as the “M/D-21”) was successfully completed from Groom Lake. Per Johnson: "Bill Park flew at the end of the day. It flew well and, in spite of having low-powered engines, went supersonic on the first flight."

A Cardinal Number is a number that says how many of something there are, such as one, two, three, four, five. An Ordinal Number is a number that tells the position of something in a list, such as 1st, 2nd, 3rd, 4th, 5th etc. Most ordinal numbers end in "th" except when the final word is:

First Flight and Service Date of the SR-71. The first flight of an SR-71 took place in December of 1964, at Air Force Plant 42 in Palmdale, California. The first SR-71 entered regular service in January of 1966. The SR-71 was the world's fastest and highest-flying operational manned airplane throughout its career. SR-71 Speed Records.

英文序數縮寫 用數字+最後兩個字母，1st、2nd、3rd，注意第11的縮寫是11th，不是11st; 若基數與序數同時出現時，先說序數再說基數，例如：The first three students will receive scholarships.(前三名的學生可以獲得獎學金) 序數對照表. 英文序數寫法是

第21 twenty-first 21st，第22 twenty-second 22nd，第23 twenty-third 23rd， 第24 twenty-fourth 24th，第25 twenty-fifth 25th. 第26 twenty-sixth 26th，第27 twenty-seventh 27th，第28 twenty-eighth 28th， 第29 twenty-ninth 29th，第30 thirtieth 30th. 第31 thirty-first 31st，第32 thirty-second 32nd，第33 thirty-third 33rd，

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1st是first的另一种写法，同理2nd是second，third 是3rd数字+th其实是本身序数词的用阿拉伯数字代表的简写，比如 fourth 是4th，所以没有1th 或者2th的写法。 知乎，中文互联网高质量的问答社区和创作者聚集的原创内容平台，于 2011 年 1 月正式上线，以「让人们更好的分享知识、经验和见解，找到自己的解答」为品牌使命。 知乎凭借认真、专业、友善的社区氛围、独特的产品机制以及结构化和易获得的优质内容，聚集了中文互联网科技、商业、影视、时尚、文化等领域最具 …

2011年8月30日 · first (1st) 、second (2nd) 、third (3rd) ，这类序数词只有三个，在整个序数词里面是特殊的，就和第一类基数词一样，需要逐个地硬记下来。 2、基数词后面加后缀-th. 这一类序数词共有十六个。均在相应的基数词后面加上后缀－th构成。