21. The development of industry has been__________ gradual process throughout__________ human existence, from stone tools to modern technology.
A.不填 ; the B. the; a C. a; 不填 D. a; a
22. Your__________ as a student will be excellent if you develop a habit of reflecting on how you learn.
A. operation B. growth C. performance D. character
23. The research lacks__________evidence, and therefore, its conclusions are doubtful.
A. solid B. fierce C. severe D. potential
