A) They may have been caused by the damage to the radio systems.
B) They may have taken place during take-off and landing.
C) They were proved to have been caused by the passengers portable computers.
D) They were suspected to have resulted from electromagnetic interference.
23.Few airlines want to impose a total ban on their passengers using electronic devices because ________.
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 fall back on
24.Why is it difficult to predict the possible effects of electromagnetic fields on an airplane‘s computers?
A) Because it is extremely dangerous to conduct such research on an airplane.
B) Because it remains a mystery what wavelengths are liable to be interfered with.
C) Because research scientists have not been able to produce the same effects in labs.
D) Because experts lack adequate equipment to do such research.
25.It can be inferred from the passage that the author ________.
A) is in favor of prohibiting passengers‘use of electronic devices completely
B) has overestimated the danger of electromagnetic interference
C) hasn‘t formed his own opinion on this problem
D) regards it as unreasonable to exercise a total ban during flight
答案：21.C 22.D 23.B 24.C 25.A