压力容器上使用的压力表的校验周期为（）。
A、一年至少一次
B、半年至少一次
C、不定期限
D、以上都不对
参考答案：B
Trees provide food and shelter and make us ______ comfortable.
A．feeling
B．feel
C．feels
D．felt
29. When I met him last year in Beijing, he was busy ___________ a new job.
A．watching over
B．suffering from
C．hunting for
D．looking after
