$Nicotiana$ $sylvestris$ flowers only during long days,while $N.$ $tobacum$ flowers only during short days. If raised in the laboratory under different photoperiods,they can be induced to flower at the same time and can be cross-fertilized to produce self-fertile offspring.
What is the best reason for considering $N.$ $sylvestris$ and $N.$ $tobacum$ to be separate species?

• A
  They are physiologically distinct
• B
  They are morphologically distinct
• C
  They cannot interbreed in nature
• D
  They are reproductively distinct