B选项的后半句its 60 square miles of…是一个独立主格结构。具体可以参考以下内容:http://grammar.ccc.commnet.edu/grammar/phrases.htm#absolute
ABSOLUTE PHRASE
Usually (but not always, as we shall see), an absolute phrase (also called a nominative absolute) is a group of words consisting of a noun or pronoun and a participle as well as any related modifiers. Absolute phrases do not directly connect to or modify any specific word in the rest of the sentence; instead, they modify the entire sentence, adding information. They are always treated as parenthetical elements and are set off from the rest of the sentence with a comma or a pair of commas (sometimes by a dash or pair of dashes). Notice that absolute phrases contain a subject (which is often modified by a participle), but not a true finite verb.
Their reputation as winners secured by victory, the New York Liberty charged into the semifinals.The two superstars signed autographs into the night, their faces beaming happily.
When the participle of an absolute phrase is a form of to be, such as being or having been, the participle is often left out but understood.
The season [being] over, they were mobbed by fans in Times Square.Another kind of absolute phrase is found after a modified noun; it adds a focusing detail or point of focus to the idea of the main clause. This kind of absolute phrase can take the form of a prepositional phrase, an adjective phrase, or a noun phrase.
The old firefighter stood over the smoking ruins, his senses alert to any sign of another flare-up.
His subordinates, their faces sweat-streaked and smudged with ash, leaned heavily against the firetruck.
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