Which of the following statements is true about gentamicin ?

• A. It is a polar cation and therefore, it is very poorly absorbed from the gastrointestinal tract
• B. It is not used in combination with a cephalosporin for the therapy of proven gram negative microbial infections
• C. Nephrotoxicity and irreversible ototoxicity are not the most important and serious side effects of gentamicin
• D. Gentamicin is effective for the treatment of pneumonia due to anaerobes