Sure, here are the answers to the questions based on the passage:

76. Who goes on the expeditions?
- Young people of different nationalities go on the expeditions.

77. How many expeditions are there every year?
- There are ten expeditions every year.

78. How long does each expedition last?
- Each expedition lasts for ten weeks.

79. What do the volunteers help scientists?
- Volunteers help scientists to do environmental research.

80. Which language must we know to become a volunteer?
- You have to speak some English to become a volunteer.