在客票上“NOT VALID AFTER”和“NOT VALID BEFORE”是什么意思？这两行字通常出现在什么类型的客票中？

（） a contract last year and it is still valid.

A:We have signed B:We signed C:We haven’t signed D:We have signed

（） a binding contract last year and it is still valid.

A:We have signed B:We haven’t signed C:We signed D:We had signed

______ a binding contract last year and it is still valid.

A:We have signed B:We haven't signed C:We signed D:We had signed

（）a binding contract last year and it is still valid.

A:We have signed B:We haven't signed C:We signed D:We had signed

Sometimes a researcher cannot get valid results because ______.

A:the information needed is too complex to be gathered B:personal interviewing is the most accurate of all survey research techniques C:personal interviewing is not as effective as other research techniques D:both interviewer and interviewee may make mistakes during the interviewing

A valid identification card can be used to ______.

A:take intercampus buses B:read announcements C:make use of the recreational facilities D:obtain the next issue of the student newspaper

______ a contract last year and it is still valid.

A:We have signed B:We signed C:We haven't signed D:We have signed

The International Oil Pollution Prevention Certificate is valid for a period of _____.

A:one year from the date of issue B:two years from the date of issue C:three years from the date of issue D:four years from the date of issue

The IOPP certificate is valid for __ _.

A:one year B:two years C:four years D:five years