On completion of the activities you will have to continue answering some questions.
Giải chi tiết:
Đáp án là B. On + V-ing = When + S + V ( đồng chủ ngữ với vế sau)
Around 150 B.C. the Greek astronomer Hipparchus developed a system to classify stars according to brightness.
Biogas can be utilized for electricity production, cooking, space heating, water heating and process heating.
Population growth rates vary among regions and even among countries within the same region.