The biggest safety threat facing airlines today may not be a

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(电磁干扰).The source of this interference remains unconfirmed（未经证实的）,but increasingly,experts 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 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,given that many passengers want to work during flights.
The difficulty is predicting how electromagnetic fields might affect an aircraft's computers.Experts know that portable devices emit radiation which affects those wavelengths which aircraft use for navigation（导航） and communication.But,because they have not been able to reproduce these effects in a laboratory,they have no way of knowing whether the interference might be dangerous or not.
The fact that aircraft may be vulnerable（易受...的攻击） to interference raises the risk that terrorists may use radio systems in order to damage navigation equipment.As worrying,though,is the passenger who can't hear the instructions to turn off his radio because the music's too loud.
8.Few airlines want to impose a total ban on their passengers using electronic devices because ____________________.
A.they have other effective safety measures to fall back on
B.most passengers refuse to take a plane which bans the use of radio and cassette players
C.they don't believe there is such a danger as radio interference
D.the harmful effect of electromagnetic interference is yet to be proved
9.Why is it difficult to predict the possible effects of electromagnetic fields on an airplane's computers?
A.Because it remains a mystery what wavelengths are liable to be interfered with.
B.Because it is extremely dangerous to conduct such research on an airplane.
C.Because experts lack adequate equipment to do such research.
D.Because research scientists have not been able to produce the same effects in labs.

8-D
9-D