Change default recaptcha challenge

My website contact page using recaptcha. And I always get audio challenge every time I enter the page. How to change recaptcha to show image challenge as a default and only change to audio challenge when user click the audio button. Why ? because the audio challenge seems not clear enough and maybe a little creepy. Thanks -- You received this message because you are subscribed to the Google Groups "reCAPTCHA" group. To unsubscribe from this group and stop receiving emails from it, send an email to recaptcha+unsubscribe@googlegroups.com. To post to this group, send email to recaptcha@googlegroups.com. Visit this group at http://bit.ly/1dkFnYd. For more options, visit http://bit.ly/P65DvS.