[单项选择]人工行走运送火药时，在巷道内行走，两人相距保持10米以上，（）在途中戏闹和逗留，并要注意来往车辆，坚持走人行道，不要随意横穿轨道。
A. 可以
B. 允许
C. 不得
D. 随意

The biggest safety threat facing airlines today may not be a terrorist with a gun, but the man with the portable computer in business class. In the last 15 years, pilots have reported well over 100 incidents that could have been caused by electromagnetic interference. The source of this interference remains unconfirmed, but increasingly, exports are pointing the blame at portable electronic devices such as portable computers, radio and cassette players and mobile telephones. RTCA, an organization which advises the aviation (航空) industry, has recommended that all airlines ban (禁止) such devices from being used during "critical" stages of flight, particularly take-off and landing. Some experts have gone further, calling for a total ban during all flights. Currently, rules on using these devices are left up to individual airlines. And although some airlines prohibit passengers from using such equipment during take-off and landing, most are reluctant to enforce a total ban,
A. they don’’t believe there is such a danger as radio interference
B. the harmful effect of electromagnetic interference is yet to be proved
C. most passengers refuse to take a plane which bans the use of radio and cassette players
D. they have other effective safety measures to fail back on