Match the missing words in the following phrase. Bacterial and archaeal species are difficult to define because the [A] definition of production of viable offspring by sexual reproduction does not apply to asexual organisms. [B] is very common among bacteria and archaea, which further confuses the species concept. Some ways that researchers have used to classify [C] species are: definition based on morphological and biochemical similarity, DNA [D] capacity, frequency of natural DNA [E], and DNA [F] within a defined range.