Let $f(t)$ denote the number of people eating in a restaurant $t$ minutes after 5 PM. Answer the following questions:

a) Which of the following statements best describes the significance of the expression $f( 4 ) = 21$?

A. There are 4 people eating at 5:21 PM
B. There are 21 people eating at 5:04 PM
C. There are 21 people eating at 9:00 PM
D. Every 4 minutes, 21 more people are eating
E. None of the above

b) Which of the following statements best describes the significance of the expression $f(a) = 20$?

A. $a$ minutes after 5 PM there are 20 people eating
B. Every 20 minutes, the number of people eating has increased by $a$ people
C. At 5:20 PM there are $a$ people eating
D. $a$ hours after 5 PM there are 20 people eating
E. None of the above

c) Which of the following statements best describes the significance of the expression $f( 20 ) = b$?

A. Every 20 minutes, the number of people eating has increased by $b$ people
B. $b$ minutes after 5 PM there are 20 people eating
C. At 5:20 PM there are $b$ people eating
D. $b$ hours after 5 PM there are 20 people eating
E. None of the above

d) Which of the following statements best describes the significance of the expression $n=f(t)$?

A. Every $t$ minutes, $n$ more people have begun eating
B. $n$ hours after 5 PM there are $t$ people eating
C. $n$ minutes after 5 PM there are $t$ people eating
D. $t$ hours after 5 PM there are $n$ people eating
E. None of the above