10
2
4
6
8
10
12
14
16
3
1980
1
1981
3
1982
5
1983
2
1984
5
1985
2
1986
0
1987
1
1988
5
1989
3
1990
4
1991
6
1992
4
1993
5
1994
4
1995
4
1996
3
1997
9
1998
5
1999
2
2000
2
2001
4
2002
7
2003
5
2004
5
2005
6
2006
5
2007
11
2008
7
2009
5
2010
16
2011
11
2012
9
2013
8
2014
10
2015
15
2016
10
2017
The Conversation, CC-BY-ND
