External pneumatic compression has been advocated for the prevention of deep venous thrombosis during operative procedures. Which of the following statement(s) concerning the use of external pneumatic compression devices is/are true? a. Intermittent pneumatic compression is as effective as low-dose heparin in prevention of venous thrombosis b. These devices function by compressing the lower extremities therefore augmenting venous return c. Pneumatic compression devices may also exhibit their antithrombotic effect through stimulating local and systemic fibrinolysis d. The length of time that intermittent pneumatic compression should be used includes through the operation and for at least several days in the postoperative period