[单选] 一名研究人员正在研究治疗肠易激综合征常用抗carinics的生物利用度。药物“A”每天口服100毫克，60毫克药物从胃肠道吸收而不发生变化。那么，药物“A”的生物利用度是（）？A researcher is studying the bioavailability of commonly used antimuscarinics to treat irritable bowel syndrome． Medication “A” is administered in a 100 mg daily dose orally and 60 mg of the drug is absorbed from the gastrointestinal tract unchanged． Thus， the bioavailability of medication “A” is（）？