What scientists understand that earth's interior is liquid?
a) p waves cannot travel through liquids.
b) p and s waves cannot travel through liquids.
c) p and s waves travel through liquids at different speeds.
d) s waves cannot travel through liquids.

The us and canadian governments are doing all of the following to combat environmental problems except a. encouraging the use of public transportation b. enacting clean water policies c. requiring emissions testing d. limiting animal populations in some regions