滚动轴承钢常用的牌号是（     ）
A、T12
B、GCr15 
C、HT350 
D、65Mn

单项选择题Read the following excerpts from different genres and determine the genres according to the language features. The data was obtained from interviewing 150 year one students about their experiences during their first month at university. The respondents were asked 20 multiple-choice questions divided in to four sections on orientation, course information, socializing and belonging. The questionnaires were administered at the end of September and the results collated the following month.

单项选择题若不使用51单片机内部存储器引脚EA必须接地。 A、正确 B、错误