Which sentence correctly uses a comma to separate the introductory phrase?
a) the library of congress is the largest library in the world, because it contains more than 147 million items.
b) the largest library in the world is the library of congress, which contains more than 147 million items.
c) the library of congress, which is the largest library in the world, houses over 147 million items.
d) containing more than 147 million items, the library of congress is the largest library in the world.

*** the correct answer is d. just those that want to know***