合同履行时，履行期限不明确的，债权人可以随时要求债务人履行。

XP is (71) of interesting twists that encourage one to think--for example, how about "Test and then code" I’ve worked with software companies and a few IT organizations in (72) programmer performance was measured on lines of code delivered and testing was measured on defects found-- (73) side was motivated to reduce the number of defects prior to testing. XP uses two types of testing: unit and functional. (74) , the practice for unit testing involves developing the test for the feature prior to writing the code and further states that the tests should be automated. Once the code is written, it is immediately (75) to the test suite bringing instant feedback.

（74）处填（）。

A．whereas

B．Although

C．However

D．And then