39 According to the passage, what does the idea that mammals had an opportunity to radiate into available niches imply? (A) Mammals struggled for millions of years in the available Mesozoic undergrowth. (B) Mammals could spread out into new territory due to the extinction of dinosaurs. (C) Mammals had the ability to emit light into the niches. (D) Mammals had to find other niches because of the radiation of the sun.