Quant question about hypothesis test

Module quiz 6.2
A researcher has 28 quarterly excess returns to an investment strategy and believes these returns are approximately normally distributed. The mean return on this sample is 1.645% and the standard deviation is 5.29%. For a test of the hypothesis that excess returns are less than or equal to zero, the researcher should:
A. reject the null hypothesis because the critical value for the test is 1.645.
B. not draw any conclusion because the sample size is less than 30.
C. fail to reject the null because the critical value is greater than 1.645.

the correct answer is B.
i think since these returns are approximately normally distributed, for this test we would choose t-test, so why does sample size less than 30 still matter? I wonder why not choose C?

What’s the confidence level for the test?

it doesn’t say

Then how can you know what the critical value is?

oh! that’s it, thanks!

My pleasure.