The following are the concentration versus time plots of the reactants and products represented by the reaction 

$$\text{L}(g) + 2\text{M}(g) \rightarrow \text{N}(g) + 3\text{O}(g)$$. 

The curves that represent M(g) and N(g) qualitatively are respectively

A

X, Y

B

Y, U

C

V, Y

D

U, X