The sky is much bluer ________ we have taken action to protect the environment.
A.because B.before C.until D.unless
Taxi Apps（打车软件）can help us travel around more easily. ___more and more people like using them.
A.Or B.So C.And D.But
I really want to have a pet dog,_____ my mother disagrees.
A. but B. and C. or D. so
Some people won't realize the importance of their health u________ they lose it.
The story isn't hard to understand, b________ there are no new words in it.
T________ Edison failed many times, he didn't give up and succeeded in the end.
