In this event-driven , double-blind study , we randomly assigned , in a 2:1:1 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 10 mg of ambrisentan plus 40 mg of tadalafil ( combination-therapy group ) , 10 mg of ambrisentan plus placebo ( ambrisentan-monotherapy group ) , or 40 mg of tadalafil plus placebo ( tadalafil-monotherapy group ) , all administered once daily .