digital circuits – What is the behavior of the given counter?

I found the following question –

For the circuit shown in the figure, the delay of the bubble NAND gate is 2ns and that of the counter is assumed to be zero.

enter the description of the image here

If the clock frequency (Clk) is 1 GHz, the counter behaves like a

(A) mod-5 counter
(B) counter mod-6
(C) counter mod-7
(D) mod-8 counter


What I can see is that, in this particular case, the clock period is less than the delay in the NAND gate. But I am unable to see how that will affect the counter exactly!