This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to breed cockatiels
Make sure your cockatiels are old enough.
For breeding purposes, your male and female cockatiel should be at least 18 months old. Younger females may experience problems like egg binding and younger birds also may not take good care of their babies.

pick from the following.
(A). Young cockatiels can become stressed or tense and may become aggressive when they develop, so you will need to keep your young in fresh air, clean, and happy. Birds also come under some stress when they are ready for a " magical vacation "..
(B). However, older bird breeds will look and feel better. These bald breeds are more aggressive and set into the pecking order of cockatiels, which means they come from a clean environment where their gender is not made too clear..
(C). You will need to be very careful that your female cockatiels have a whole litter of eggs, as this could be harmful for your bird species. If you do not have old hens, think about using commercial eggs..
(D). Egg binding is a condition where the eggs become stuck in the cloaca. This may lead to an infection and even death..
The answer is:
(D).