1 the => bỏ
2 planting => being planted
3 greener => greenest
4 problem => problems
1. Last week, we traveled to our hometown by the train.
the=>bỏ
2. Many trees were planting when we visited the children.
planting=>planted
3. Stockholm is one of the greener cities in the world.
greener=>greenest
4. One of the most serious problem now is the spread of COVID-19.
problem=>problems