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?