Which American English sound is often considered the most difficult for non-native speakers to pronounce?
The /r/ sound, as in 'red' and 'car'
The /ʃ/ sound, as in 'shoe' and 'sugar'
The /ɑ/ sound, as in 'father' and 'palm'
The /θ/ sound, as in 'thin' and 'mouth'

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