Thank you so much!

What helped me understand the sentence was noticing that

A is much more closely related to B than to C

- In this case, A is some cultivars of corn. B is sorghum. C is most cultivars of corn.

The sentence explicity told us A is more closely related to B. So, if there is anything for us to problem solve, it must be the relationship between B and C. If this is the case, then what quality are we comparing on? A (some cultivars of corn).

But what about some cultivars of corn? We need a more specific trait? It's color, it's density, its growth rate? No, the sentence tells us it's morphological relativity.

So, between B and C, which is more morphologically related to A?

B, sorghum is.