An excellent teacher is believed to be one who devotes (奉献) himself to the children but asks for nothing         .

The filling station _____ because there were too many others on the street.

Every student is required to watch the teacher carefully before ___ the chemistry experiment.

Some insects_____ the colour of their surroundings (环境) to protect themselves.

Many political leaders attended the conference and among those present ____ the foreign minister and his wife.