一堆钢管，最上层有2根，最下层有12根，相邻两层相差1根，这堆钢管一共有（ ）根。
77
下列哪些属乙类传染病（ ）
A．病毒性肝炎
B．细菌性疾病
C．艾滋病
D．伤寒
E．肺结核
The research mainly deals with the difficulty the students have ________ their attention on their study all
the time.[ ]
A．to focus
B．to be focused
C．focusing
D．focused
