Around the year 1130,the Native American Anasazi tribe disap

Around the year 1130,the Native American Anasazi tribe disappeared.Although several theories have been developed,scientists remain uncertain about _______ to them.
A:happened B:that happened
C:happening D:what happened
Pierre is a long-distance cyclist.He always carries bottled water or fortified beverages when he rides.It is essential that he _______ plenty of fluids during his long activity so that he won't become dehydrated.
A:drink B:drank
C:will drink D:are drinking
Amelia Earhart attempted to circumnavigate the globe in 1937.________ was the reason she embarked upon her journey.She wanted to be the first aviator to do it.
A:That it had never been done before
B:No one had done it before
C:It had never been done before
D:Never been done before
Our teacher encouraged everybody's participation in the debate,but he insisted that everyone _______ respect for the opinions of others.
A:show B:showed
C:showing D:shows
_______ no money,but that didn't bother him at all.He knew he'd find a job soon.
A:James had B:That James had
C:Why James had D:Why did James have