help with SAT writing section

Question

Scientists have long believed that caels originated in North America and then spread throughout the world, a theory that was first proposed after a camel fossil was discovered in Canada's Yukon Territory in 1913. A) No Change B) this first being proposed C) this theory was first proposed D) and the first theory proposed

I had thought the answer would be B), rather than A) although A) too sounds correct. However, the answer apparently is indeed A). However, I don't understand why I should choose A) over B)... Could someone help me understand this, please?

Answer 1

Suggestion B) would read as:

Scientists have long believed that camels originated in North America and then spread throughout the world, this first being proposed after a camel fossil was discovered in Canada's Yukon Territory in 1913.

In the sentence it is unclear to what this refers. Also this first being could refer to the camel or to a scientist (a being is a creature).

But in the given sentence it is clear that the origin of camels is a theory, and why the theory came about.