pauseSpeakingAtBoundary no effect

Here is my problem , I can't understand why AVSpeechSynthesizer pause no effect.

I have a AVSpeechSynthesizer object, and this object have 5 uttrances in its queue.

so , i do like this:

when the first uttrance in the queue play finished, i call AVSpeechSynthesizer pauseSpeakingAtBoundary method immediately, but there is no effect , the second uttrance still will be played.

my code like this:

- (void)beginConversation {
    for (NSInteger i = 0; i < [_speechStrings count]; i++) {
        AVSpeechUtterance *uttrance = [AVSpeechUtterance speechUtteranceWithString:[_speechStrings objectAtIndex:i]];
        uttrance.voice = [_voices firstObject];
        uttrance.rate = AVSpeechUtteranceMaximumSpeechRate * 0.5;
        uttrance.pitchMultiplier = 1.0f;
        [_synthesizer speakUtterance:uttrance];
    }

}

// button Click to pause or resume AVSpeechSynthesizer
- (void)pauseOrPlaySpeech {
    AVSpeechSynthesizer *synthesizer = _speech.synthesizer;
    if ([synthesizer isPaused]) {
        if (![synthesizer continueSpeaking]) {
            NSLog(@"continue speaking error");
        }
    } else {
        if ([synthesizer isSpeaking]) {
            if (![synthesizer pauseSpeakingAtBoundary:AVSpeechBoundaryWord]) {
                NSLog(@"pause speaking error");
            }
        } else {
            [_speech beginConversation];
        }
    }
}