Which of the following statements is correct?

• A
  $E. coli$ is a Gram-negative bacterium, while $Rhizobium \text{ } japonicum$ is a Gram-positive bacterium.
• B
  Both $E. coli$ and $Rhizobium \text{ } japonicum$ are Gram-negative.
• C
  Both $E. coli$ and $Rhizobium \text{ } japonicum$ are Gram-positive.
• D
  $E. coli$ is a Gram-positive bacterium, while $Rhizobium \text{ } japonicum$ is a Gram-negative bacterium.