d regions in a sentence

1. Sky-wave ionospheric propagation is not possible during the day because of the attenuation of the signals on these frequencies caused by the D region in the ionosphere.
2. When meiosis tries to do recombination between the strands at the C-D region, you end up with some number of A B C D E B A and C D E chromosomes, which tend not to be viable.
3. It's difficult to find d regions in a sentence.