Lions and tigers sometimes breed in zoos, but hybrids have not been seen in nature and even in zoos they do not reproduce well. Which of the following statements is the best conclusion that can be derived from this information?

A) The biological species concept is invalid because hybrids can be formed.
B) Lions and tigers are not actually different species because they can produce viable offspring in captivity.
C) The morphological or ecological species concepts must be used rather than the biological species concept because there could be gene flow between the species in nature.
D) The biological species concept can be applied because species are, in part, defined by the inability to produce viable, fertile offspring with other species.