单项选择题

In vacuum a current I flows through the wire which is bent into a semicircular section and two long straight sections. Magnetic field at point O equals( ).

A、0
B、
C、
D、

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单项选择题

15. He waited for _______the bus. A.
long time B.
a long time C.
some long time D.
the long time

单项选择题

A．①
B．②
C．③
D．④