Which of the following statements about glycosaminoglycans are true?

$($a$)$ They contain derivatives of either glucosanine or galactosamine:

$($b$)$ They constitute $5\%$ of the weight of proteoglycans.

$($c$)$ They contain positively charged substituent groups.

$($d$)$ They include heparin, chondroitin sulfate, and keratan sulfate.

$($e$)$ They have repeating units of four sugar groups.