Why are certain amino acids called essential amino acids?

Which of the following lines of evidence would best support your assertion that a particular plant is an angiosperm? a) it produces seeds. b) it retains its fertilized egg within its archaegonium. c) it lacks gametangia. d) it undergoes alternation of generations.

One function of the poly-a tail on eukaryotic mrna sequences is to the mrna be transported from the nucleus to the cytoplasm. prokaryotic mrna also has a poly-a tail. choose the best explanation of the prokaryotic poly-a tail. a. prokaryotic poly-a tails are composed of a different molecular structure compared with eukaryotic poly-a tails. b. prokaryotic poly-a tails have the same functions as eukaryotic poly-a tails, because this process is highly conserved throughout different species. c. prokaryotic poly-a tails aren't important, because prokaryotes don't have nuclei. d. prokaryotic poly-a tails have other functions,because prokaryotes don't have nuclei.

What is the difference between a scientific hypotheses and a scientific theory?