问题详情：
![—IheardthatmanypeopleplantedtreesonTreePlantingDaywitho...]( "—IheardthatmanypeopleplantedtreesonTreePlantingDaywitho...")
—I heard that many people planted trees on Tree Planting Day without stepping out this year. I wonder ▲ .
—They did this by taking part in“cloud planting" on the Internet.
A. what they did B. how they did that C. why they joined the activity
【回答】
B
知识点：宾语从句
题型：选择题
