Two vectors: no. Three vectors: yes.
by vector addition?
true
Two vectors with unequal magnitudes can't add to zero, but three or more can.
As the velocities are in the same direction then addition of vectors becomes so easy. We simply add the magnitudes of the velocities. If velocities go exactly opposite, then we get the difference of their magnitudes. If velocity vectors get inclined, then we use the parallelogram law of vectors to get the resultant.
No
Two vectors: no. Three vectors: yes.
by vector addition?
Yes.
true
Two vectors with unequal magnitudes can't add to zero, but three or more can.
Yes, put the three vectors in a plane, with a separation of 120 degrees between each vector and each of the other vectors.
mAYBE
No.
True
By all means. Using Newton's third law (f=ma) Multiply the vectors by 0 and you're home
If none of the individual vectors has a magnitude of zero, thenthe minimum number that can combined to make zero is two.