og 19-792关于and前面逗号的问题

crystal_10

Scientists have recently found evidence that black holes—regions of space in which matter is so concentrated and the pull of gravity so powerful that nothing, not even light, can emerge from them—probably exist at the core of nearly all galaxies and the mass of each black hole is proportional to its host galaxy.

• A
  
  exist at the core of nearly all galaxies and the mass of each black hole is proportional to
  
  
  
• B
  
  exist at the core of nearly all galaxies and that the mass of each black hole is proportional to that of
  
  
  
• C
  
  exist at the core of nearly all galaxies, and that the mass of each black hole is proportional to
  
  
  
• D
  
  exists at the core of nearly all galaxies, and that the mass of each black hole is proportional to that of
  
  
  
• E
  
  exists at the core of nearly all galaxies and the mass of each black hole is proportional to that of
  
  
  
  

正确答案是B选项。请问加逗号和不加逗号有啥区别呢？