——Tom isn't feeling very well.
——He seems to have a cold _________ the weather changes suddenly.
A:before
B:so
C:because
D:though
生物之所以能保持种族性状的相对稳定,是因为生物具有 ( )
已知fx=xlnx,若f'x0=2,则x0等于 ( )
若4a2﹣(k﹣1)a+9是一个关于a的完全平方式,则k=____