The essential amino acids cannot be produced in the human body and hence these amino acids have to be obtained through the diet. Hence these amino acids have this characteristic that these cannot be produced in the human body and have to be obtained through diet this makes them essential. Hence those nine amino acids are considered as the essential amino acids.
Your answer is A) - a greenhouse effect
that happens also at large scale, when the atmosphere is playing the role of the windows and the climate on earth became warmer.
The answer would be: C) sexual selection.
Sexual selection happens where one of the individuals choose the partner to sex based on some criteria. In this case, the ability to have a beautiful bower will determinate the ability of the bird to have sex and produce offspring. The criteria are not limited to visual. It could be diverse in other cases, like a deeper croak in frog
