Which of the Following Is True of Tongue-Retaining Devices (TRDs)used

Which of the following is true of tongue-retaining devices (TRDs) used to treat patients with sleep-related breathing disorders?

A) They typically are made of acrylic resin and are composed of two pieces that cover the upper and lower dental arches.
B) Over 70 different types of TRDs have been approved by the FDA for treatment of obstructive sleep apnea (OSA) .
C) TRDs are capable of providing continuous positive airway pressure (CPAP) .
D) TRDs generally are made of silicone in the shape of a bulb or cavity.