Help a Thief!!!

You have to help a thief to steal as many as GoldCoins as possible from a GoldMine. There he saw N Gold Boxes an each Gold Boxes consists of Ai Plates each plates consists of Bi Gold Coins. Your task is to print the maximum gold coins theif can steal if he can take a maximum of T plates.

Example 1:

Input:
T = 3, N = 3
A[] = {1, 2, 3}
B[] = {3, 2, 1}
Output:
7
Explanation:
The thief will take 1 plate of coins
from the first box and 2 plate of coins
from the second plate. 3 + 2*2 = 7.
Example 2:

Input:
T = 0, N = 3
A[] = {1, 3, 2}
B[] = {2, 3, 1}
Output:
0
Explanation:
The thief can't take any plates.
So he can't steal any coins.

maxCoins() which takes 2 Integers T, and N and two arrays A and B of size N as input and returns the maximum number of gold coins the thief can steal if he can take a maximum of T plates.

In the above question I thought we can use Knapsack pattern but to my surprise greedy solution worked for this problem. My question is why greedy solution works here?

Someone please explain!?