Which is an exponential decay function? f start bracket x end bracket equals three-fourths start bracket seven-fourths end bracket superscript x f start bracket x end bracket equals two-third start bracket four-fifths end bracket superscript negative x f start bracket x end bracket equals three-halves start bracket eight-sevenths end bracket superscript negative x f start bracket x end bracket equals one-third start bracket negative nine-halves end bracket superscript x