Critical path - help

[nbalakers24]

Question 6
A particular design project has six activities, these being A, B, C, D, E and F. The latest starting time
for activity D is day 4. If it is known that activity D lies on the critical path for this project, then the
earliest starting time for this activity is
A Day 1
B Day 2
C Day 4
D Day 5
E Not able to be determined

How do you work it out? 
The answer is C but i got E LOL

Cheers!!

and heres what they say in the solutions:

Question 6 [C]
For an activity to be critical, the earliest and latest
start times will be equal.

[nbalakers24]

nvermind. i finally understand it. LOL