iOS 如何让音频文件不要同时播放。

下列代码SongA 与SongB同时播放了，有没有办法让它们不要同时播放。


NSString *sOngA= [[NSBundle mainBundle] pathForResource:@"songA" ofType:@"mp3"];
NSError *soundError = nil;
self.player = [[AVAudioPlayer alloc] initWithContentsOfURL:[NSURL fileURLWithPath:songA] error:&soundError];
if(self.player == nil)
NSLog(@"%@",soundError);
else
{
[self.player setDelegate:self];
[self.player setVolume:0.75];
[self.player play];
}

NSString *sOngB= [[NSBundle mainBundle] pathForResource:@"songB" ofType:@"mp3"];
soundError = nil;
self.player = [[AVAudioPlayer alloc] initWithContentsOfURL:[NSURL fileURLWithPath:songB] error:&soundError];
if(self.player == nil)
NSLog(@"%@",soundError);
else
{
[self.player setDelegate:self];
[self.player setVolume:0.25];
[self.player play];
}