再问GWD-9-40

robertchu

"All the recorded staple crops grown in western Africa were introduced from elsewhere, beginning, at some unknown date, with rice and yams."

The keyword is "recorded".  It could be the case that some staple crops grown there were somehow not recorded.

BTW, please always post the complete answer choices when asking questions.  And, since this question has been discussed before, it's usually better to simply follow up in the original thread.