From Past Records It Is Known That the Average Life

From past records it is known that the average life of a battery used in a digital clock is 305 days.
The battery life is normally distributed. The battery was recently modified to last longer. A sample of
20 of the modified batteries was tested. It was discovered that the mean life was 311 days and the
Sample standard deviation was 12 days. We want to test at the 0.05 level of significance whether
The modification increases the life of the battery. What is our decision rule?

A) Do not reject the null hypothesis if computed t is 1.96 or greater
B) Reject the null hypothesis if computed t is less than 1.96
C) Do not reject the null hypothesis if computed t is 1.729 or greater
D) Reject the null hypothesis if computed t is 2.086 or greater
E) Reject the null hypothesis if the computed t is 1.729 or greater