Our teacher entered the classroom, __________.[ ]

Sam remembered several occasions in the past _____ he had experienced a similar feeling.

Do you know the young lady _____ your mother is talking

The worsening ofglobal warming, _____human activities are to blame, has led to frequentlyextreme weather.

uth-East Asia, many farmers cannot afford _____ farm equipment such as tractors, so most of the work is done with human labour.