In this event-driven , double-blind study , we randomly assigned , in a @:@:@ ratio , participants with World Health Organization functional class II or III symptoms of pulmonary arterial hypertension who had not previously received treatment to receive initial combination therapy with @ mg of ambrisentan plus @ mg of tadalafil ( combination-therapy group ) , @ mg of ambrisentan plus placebo ( ambrisentan-monotherapy group ) , or @ mg of tadalafil plus placebo ( tadalafil-monotherapy group ) , all administered once daily .