MA121: introduction to statistics/ help with problem

Hi,
I am stuck on this problem. Can anyone tell me what I am doing wrong?
100 people from a city were surveyed and 40% of the people were in favor of a new retirement plan. what is the upper limit of the 95% confidence interval of the population proportion?

my solution which is wrong is: sqrt of p(1-p)/n. I multiply the answer by 1.960 and add to p.
SQRT .4(1-,4)/100=.0489897949, .0489897949*1.960=.0960199979+.4= .4960199979
My answer is not an option. What am I missing?

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