21. Which of the following descriptions about cholesterol synthesis and metabolism is CORRECT? (A) The rate limiting step of cholesterol synthesis is regulated by HMG-CoA synthase. (B) Cholesterol transported with lipoprotein in the blood circulation are mostly as free form cholesterol, while the cholesterol stored in adipose tissue are esterified by long chain fatty acids. (C) Insulin activates HMG-CoA reductase by promoting its dephosphorylation. (D) Monoclonal antibody against proprotein convertase subtilisin/kexin 9 (PCSK9) lowers serum LDL cholesterol by enhancing the degradation of the LDL receptor. (E) Increased amount of cholesterol activates transcription factor sterol regulatory element-binding protein (SREBP) to increase HMGCR (Gene of HMG-CoA reductase) mRNA transcription.