An idea or principle that explains something.
A. concept
B. factor
C. benefit
D. impact

Random Topics:
Conditionals / Unreal PastModal AdverbsTenses and ComparisonsTenses and Word FormationTenses - Irregular VerbsPerfect Progressive TensesCountable & Uncountable NounsSentence Adverbs & Third ConditionalDirect-Indirect SpeechActive-Passive VoicesOther quiz:
Adverbs of Time and Place › ViewWhat adverb is used in the sentence “The book is upstairs.”?
A. Far
B. Downstairs
C. Upstairs
D. Always
Grammar › View
The bus was full.We couldn’t __________________
A. get along
B. get away
C. get out
D. get on
Grammar & Vocabulary › ViewWhich of the following is a detail from the section “A New Lab”?
A. He was allowed to set up a science lab in a baggage car until an accidental fire ended his experiments.
B. There, Edison worked tirelessly for many hours each day, and many of his employees worked just as hard.
C. After a few years, Thomas Edison decided that he wanted to become a full-time inventor.
D. Sitting in the dark for a few moments, people could think about the great changes Thomas Edison had made in their lives.
Past Perfect and Past Perfect Continuous Tense › View
I was really relieved when I found the documents. I _________ (look for) them whole morning.
A. had looked for
B. had been looked
C. had been looked for
D. had been looking for
