Answer: option A
Step-by-step explanation: We can construct a (1 - α) % confidence interval for mean by using the formulae below.
u = x + critical value × (standard deviation /√n)
Where x = sample mean and n is the sample size.
The critical value that we are going to use (either z or t critical values) depends on the sample size.
If n > 29 we make use of a z test and if n < 29 we make use of a t test.
Also when using a z test we will (sometimes) make use of the population standard deviation.
And when using a t test, we make use of the sample standard deviation.
"Based on last year's book sales, we estimate that the standard deviation of the amount spent will be close to $30"
Judging by the sentence above, we can see that last year books sales is a fraction of several years of book sales, hence the value of standard deviation given is a sample standard deviation.
Since we are making use of a sample standard deviation, we will be using a t test for our critical value and hence sample size must be less than 29.
These points validates option A
