Understanding Society User Support

Dear Sir/Madam,
I am writing to ask a question regarding the DK and prefer not to say options in web component of Understanding Society. I have noted that the questionnaire includes items regarding voting (e.g. which party have you voted in the general elections) and income, and those questions are asked to some sample members throught CAWI. I could not find in the data documentation information on whether the DK and Prefer not to say option are visible to the respondent. I am inferring from the questionaire and associated literature (i.e. Al Baghal and Lynn, 2015), that the DK/PNS options are not visible but do only appear once the respondent attempts to skip the survey question. Could you please confirm whether this is the case?
In case the information is available in the questionnaire and I did not see it, I do apologise for the inconvenience that this question may have posed,
BW
AG

Reference
Al Baghal, T., & Lynn, P. (2015). Using motivational statements in web-instrument design to reduce item-missing rates in a mixed-mode context. Public Opinion Quarterly, 79(2), 568-579.