challenge: Why is rubidium-87 preferred over rubidium-85 for making Bose-Einstein condensates in dilute atomic gases?
  
  A. Rubidium-87 is easier to produce than rubidium-85
  B. Rubidium-87 has a positive scattering length, making it mutually repulsive at low temperatures
  C. Rubidium-87 is more abundant than rubidium-85
  D. Rubidium-87 has a negative scattering length, making it more attractive at low temperatures
  Answer: 