Final answer:
The number of sick birds increased by a factor of 3 each week.
Step-by-step explanation:
The number of sick birds increased by a factor of 3 each week.
To understand this, let's analyze the function f(x) = 3(4). The number of birds infected with the illness is represented by f(x), where 'x' is the number of weeks after the first birds became sick. The function shows that for each week, the number of sick birds is 3 times the previous week's number.
For example, if in the first week there are 4 sick birds, in the second week there would be 3 times 4 = 12 sick birds. In the third week, there would be 3 times 12 = 36 sick birds, and so on.
Therefore, the correct answer is C. The number increased by a factor of 3 each week.