Creat as Newton was, many of his ideas        today and are being modified by the work of scientists of

our time.    [ ]

A.   have challenged        

B.   have been challenged    

C．have questioned      

D.   have been questioning

次黄嘌呤-鸟嘌呤磷酸核糖转移酶（HGPRT）是嘌呤合成补救途径的关键酶。酶活性缺乏是下列哪种疾病的主要原因（）

A.痛风

B.Lesch-Nyhan综合征

C.乳清酸尿症

D.严重联合免疫缺陷症

E.Tay-Sachs病