求助英语大神 一道英语选择题！

此题考点是非谓语动词。也算是英语语法十大重点之一了，我当年就听不懂，不过道理是这样的。
非谓语动词（哪些动词后必须接动名词或/和动词不定式作宾语；如动词后即可跟动名词又可接动
词不定式作宾语，其区别是什么, 如: stop, remember, forget, regret, go on, try, need, want, ；作定语；分词的独立主格作状语；动词不定式完成式在句中作复合谓语结构; let, make have, see, hear, look at等动词后面接不带to不定式作宾补,但在被动语态中作为主语补足语的不定实则要带to; 常跟不定式作定语的名词中, 有一部分是由常跟不定式作宾语的动词和常与不定式一起构成复合谓语结构的形容词转化或派生而来的;不定式的逻辑主语:for 或 of + 名词; do nothing/anything/everything but/except + 不带to的动词不定式; see, hear, feel, notice, watch等感官动词即可跟现在分词也可跟不定式作复合宾语,二者的区别是什么; 非谓语动词的”体”式和语态）

1) Having finished his term paper before the deadline, it was delivered to the professor before the class.
3) Harry’s advisor persuaded him taking several courses which did not involve much knowledge of mathematics.
4) Doctors often study film records of complex operations for improving their own knowledge and skill.
5) The actual quantity of folic acid(叶酸) requiring in the daily diet is not known.
6) The international law of the Sea Conference is an attempt of resolving major differences among countries with conflicting interests.
7) Harvard College was one of the first institutions of higher learning to be established in the colonies.
8) In many states, the law forbids citizens from carrying pistols or rifles without first having obtained a special permit.
19) No, I don’t think Byron is buried here. Queen Victoria thought he was so immoral to be buried in the Abbey.
20) Have you read in the Columbia Spectator that Jeff’s leg was broken while playing football? 21) The Teacher’s lecture on the past participle was three hours long and Conrad felt very boring.

obtain动作显然早于carry， 故obtain用完成式。

四个选项里只有D表示已经获得许可。

首先排除B,C
B:将来时，C：被动式，不对，人持有许可证，应该是主动（持有）形态。
A，现在进行时，意思是现在正在获得持枪许可证。
D，完成进行时，意思说明现在这个状态，已经获得了持枪许可证。
显然，句子的意思是，要先拿到持枪许可证，才允许持枪。
所以选D。

这里用的是without 加现在分词的结构。这是固定搭配。那么符合条件的有。A，C，D。
我们看C，因为这里修饰的是citizens，表示他们发出的动作，所以不能用被动语态。
而A跟D，这里要强调的是动作已经发生。整句话的意思是市民必须“先取得了”许可才能带武器。所以应该用完成时，这就是为什么用having obstained，而不是obtaining。