I believe that the answer are 3, 4, and possibly 5 because the 1st question says, ''Where on earth''. It should have been, ''When on Earth''. The others make sense because the scientist or your teacher can test it. Their has to be evidence to support it.
#1 can't be tested because it was in the last decade (which was 10 years ago). Also, it says in #1 that, Where on Earth had a population decrease.
Hope this helps... :)
-Justin0208