If the Rice Already Had the Genes That Could Make

If the rice already had the genes that could make vitamin A, why did scientists use genes from other organisms?

A) The scientists didn't know how to "turn on" the genes in the rice.
B) It's easier to introduce genes from one species into another than from just one species.
C) The rice genes didn't make the right type of vitamin A.
D) Scientists could have just "turned on" the rice genes, but they wouldn't have learned anything from that process.
E) It's easy to extract genes from bacteria.