Tom isn't Chinese. He's _______ English boy.[ ]
A. a
B. an
C. the
D. /
答案:B
灰平衡是指C、M、Y三原色以不同比例混合而形成()
哪种氮肥最适宜作根外施肥?各种作物容许喷施的尿素浓度是多少?