Marine biologists study the number of manatees killed by powerboats in florida and warn that the growing number of powerboats registered in florida threatens the existence of manatees. here are the data where powerboat numbers are in thousand.

y ear 1982 1983 1984 1985 1986 1987 1988 1989
manatee killed 13 21 24 16 24 20 15 34
powerboat 447 460 481 498 513 512 527 559
year 1990 1992 1993 1994 1995 1996 1997 1998
manatee killed 33 33 39 43 50 47 53 38
powerboat 585 614 646 675 711 719 716 716

mean standard deviation correlation coefficient (r)
manatee killed 42.7826 23.1143 0.9461
powerboat 672.7391 166.3188

what are the explanatory and response variables in this study?