空气中的负离子对人的健康极为有益。人工产生负离子的最常见方法是电晕放电法。如图所示，一排针状负极和环形正极之间加上直流高压电，电压达5000V左右，使空气发生电离，从而产生一价的负氧离子排出，使空气清新化，针状负极与环形正极间距为5mm，它们之间的电场可视为匀强电场，电子电量e=1．6×10-19C；若电场强度为E，电场对负氧离子的作用力为F，则 ( )
| A．E=103N／C， F=1.6×10-16N |
| B．E=106N／C， F=3.2×l0-13N |
| C．E=103N／C， F=3.2×10-16N |
| D．E=106N／C， F=1.6×10-13N |
In our English class，many chances_______ to students to show themselves.（ ）
| A． |
offer |
B． |
are offered |
| C． |
offered |
D． |
are offering |
﹣﹣﹣Sandy，could you tell me _______ the beautiful hair clip？
﹣﹣﹣Oh，I bought it in a shop near the school.（ ）
| A． |
where you will buy |
B． |
where will you buy |
| C． |
where did you buy |
D． |
where you bought |
I_______ books in the library when the rainstorm came.（ ）
| A． |
read |
B． |
reads |
| C． |
am reading |
D． |
was reading |
﹣﹣﹣_________ your hands before dinner，Tony.
﹣﹣﹣No problem，Mom.（ ）
| A． |
Wash |
B． |
Washes |
C． |
Washing |
D． |
To wash |
﹣﹣﹣Is Helen here？
﹣﹣﹣No，she isn't here.She_______ in half an hour.（ ）
| A． |
arrives |
B． |
arrived |
| C． |
will arrive |
D． |
has arrived |