The old couple have been married for 40 years and never once _______ with each other.
C
本题考查的是倒装结构的用法。第二分句中因有否定副词never提前，故应用倒装结构;第一分句中的havebeenmarried已限定了时态，据此可排除D项。
The basic unit in the study of morphology is ______.
He talked a lot about things and persons ________they remembered in the school.
Theothertimingsystembelongsinourinternalclocks，which，left________，wouldtiethebodytoa25-hourrhythm.
Thereisadeadlock(僵局)inthediscussionwhenneithersidegives________totheother.