How to determine which one would not be the resultant? i had a physics exam yesterday, which was all in all pretty good except for this one question which i just don't get.:

9) Two forces 4N and 6N act at a point. All of the following could be the magnitude of their resultant except.
A)   1N      B)  6N  C)10N  D)4N    E) 8N

Could you please help me to understand the logic behind this question?
 A: Given two forces, the largest magnitude their sum (resultant) can have is the sum of their magnitudes (this occurs when they point in the same direction) while the smallest magnitude their sum can have is the difference of their magnitudes (when they point in opposite directions).  
Therefore, in your example, the largest possible resultant magnitude is $6\,\mathrm N + 4 \,\mathrm N = 10 \,\mathrm N$.  None of the answer choices violates this upper bound.
Now think about whether one of the choices violates the lower bound described above.
A: It is a) 1N.
The minimum resultant force acting on the object is if they both go opposite ways; 6-4=2N ,  therefore it is absolutely impossible to get 1N as a resultant force.
A: Well, it will depend on the angles between the two given vectors. Let the angle between them be $\theta$ . So the resultant say R(only modulus)  is given by $R^2$ = $6^2$ + $4^2$ + $2*6*4$ cos$\theta$.
$\Rightarrow$ $R^2$ = 36+16+48cos$\theta$ .
$\Rightarrow$ cos$\theta$ = ($R^2$-52)/48.
Now we know, -1 $\leqslant$ cos$\theta$ $\leqslant$ 1.
$\Rightarrow$ -1 $\leqslant$ ($R^2$-52)/48 $\leqslant$ 1.
$\Rightarrow$ 2 $\leqslant$ R $\leqslant$ 10.
Now look at the options given. You will find the answer. 
A: Think about the Triangle law of addition of 2 Vectors. 
The triangle law states, 
||a| - |b|| =< | a + b | =< |a| + |b|
where a,b,c are vectors.. 
Now If a= 4 and b=6
2 =< | a + b | =< 10
Edits are welcome!
A: When the two vectors are in the same direction then they give the maximum magnitude and when they are opposite the magnitude of resultant would be minimum. We can get different values of magnitudes by changing the angle between the vectors but that will not be more or less than the above described. It can be understood with the help of assumption that our vectors are on x axis. Their y component will be zero and all they have would be their x components. Which would either cancel out each other or support depending upon how they are oriented
