关于PREP语法笔记的解释(疑问PREP2-35)

35.
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Unlike the steam locomotive, which required an hour or two of firing up before it was possible to move under its own power, nearly full power could be summoned almost instantly from the cold engine of a diesel locomotive.

(A) it was possible to move under its own power, nearly full power could be summoned almost instantly from the cold engine of a diesel locomotive

(B) it was possible to move under its own power, the diesel locomotive's engine, when cold, could produce nearly full power almost instantly

(C) it could move under its own power, the diesel locomotive could summon nearly full power from a cold engine almost instantly

(D) moving under its own power, the cold engine of the diesel locomotive could produce nearly full power almost instantly

(E) moving under its own power, almost instantly a diesel locomotive could summon nearly full power from a cold engine

PREP语法笔记说:  指代一致，形式主语it指代了句子的真正主语（真正的主语在句子后部），因为有所指，所以句子中如果再出现其它有所指的it，就冲突了。 这个是绝对没有错的,很多语法题目都是利用这一点来考察的.

对A和B选项的分析是这样的:

(A) 形式主语it与its指代对象不一致；unlike连接比较对象不对等，locomotive与power。
(B) 形式主语it与its指代对象不一致；unlike连接比较对象不对等，locomotive与engine。

但是我觉得这里的IT并不是形式主语,而是和后面ITS一样,就是指代the steam locomotive,因为我们看正确选项C, it could move under its own power,其实A,B和C里的IT指代的都是the steam locomotive,而不是充当形式主语.

但是还有一个问题是 是否有这样的形式 sth is possible to do sth, 还是只有it is possible for sb to do sth, 如果是这样的话解释就没有什么问题啦！

请NN指教！

3. 有些形容词（如possible）作表语时，通常不用"人"作主语。

我们不说He is possible to attend the meeting.         

而应该说It is possible for him to attend the meeting.

这类形容词常见的有： possible, improbable, convenient, inconvenient等
注意：这类句型中当动词不定式与句子主语存在着逻辑动宾关系时，主语用"人"是可以的，如
   He is impossible to teach. = It is impossible to teach him.
   下句也是正确的：
   She is necessary to get along well with.

上网查到这样一段资料,但是没有查字典,好象POSSIBLE确实是有问题的。对高中语法没有记忆了……