Car M traveled at a constant speed for a given time. This is shown by the dashed line. Car N traveled at twice the speed for the same distance. If Car N's speed and time are shown as solid line, which graph illustrates this? 

[asy]
unitsize(12);
draw((0,9)--(0,0)--(9,0));
label("time",(4.5,0),S);
label("s",(0,7),W); label("p",(0,6),W); label("e",(0,5),W); label("e",(0,4),W); label("d",(0,3),W);
label("(A)",(-1,9),NW);
draw((0,4)--(4,4),dashed); label("M",(4,4),E);
draw((0,8)--(4,8),linewidth(1)); label("N",(4,8),E);
draw((15,9)--(15,0)--(24,0));
label("time",(19.5,0),S);
label("s",(15,7),W); label("p",(15,6),W); label("e",(15,5),W); label("e",(15,4),W); label("d",(15,3),W);
label("(B)",(14,9),NW);
draw((15,4)--(19,4),dashed); label("M",(19,4),E);
draw((15,8)--(23,8),linewidth(1)); label("N",(23,8),E);
draw((30,9)--(30,0)--(39,0));
label("time",(34.5,0),S);
label("s",(30,7),W); label("p",(30,6),W); label("e",(30,5),W); label("e",(30,4),W); label("d",(30,3),W);
label("(C)",(29,9),NW);
draw((30,4)--(34,4),dashed); label("M",(34,4),E);
draw((30,2)--(34,2),linewidth(1)); label("N",(34,2),E);
draw((0,-6)--(0,-15)--(9,-15));
label("time",(4.5,-15),S);
label("s",(0,-8),W); label("p",(0,-9),W); label("e",(0,-10),W); label("e",(0,-11),W); label("d",(0,-12),W);
label("(D)",(-1,-6),NW);
draw((0,-11)--(4,-11),dashed); label("M",(4,-11),E);
draw((0,-7)--(2,-7),linewidth(1)); label("N",(2,-7),E);
draw((15,-6)--(15,-15)--(24,-15));
label("time",(19.5,-15),S);
label("s",(15,-8),W); label("p",(15,-9),W); label("e",(15,-10),W); label("e",(15,-11),W); label("d",(15,-12),W);
label("(E)",(14,-6),NW);
draw((15,-11)--(19,-11),dashed); label("M",(19,-11),E);
draw((15,-13)--(23,-13),linewidth(1)); label("N",(23,-13),E);[/asy]
1. **Understanding the problem:**
   - Car M travels at a constant speed for a given time.
   - Car N travels at twice the speed of Car M for the same distance.
   - We need to determine which graph correctly represents the speed and time of Car N compared to Car M.

2. **Using the formula \( d = rt \):**
   - Let \( r_M \) be the speed of Car M and \( t_M \) be the time Car M travels.
   - The distance \( d \) traveled by Car M is given by:
     \[
     d = r_M \cdot t_M
     \]
   - Car N travels at twice the speed of Car M, so \( r_N = 2r_M \).
   - Car N travels the same distance \( d \), so:
     \[
     d = r_N \cdot t_N = 2r_M \cdot t_N
     \]
   - Since the distances are equal, we have:
     \[
     r_M \cdot t_M = 2r_M \cdot t_N
     \]
   - Solving for \( t_N \):
     \[
     t_N = \frac{t_M}{2}
     \]

3. **Analyzing the graphs:**
   - We need to find the graph where Car N (solid line) travels at twice the speed of Car M (dashed line) but takes half the time to travel the same distance.

4. **Checking each graph:**
   - **Graph (A):** Car N has the same time as Car M but a higher speed. This does not match our condition.
   - **Graph (B):** Car N has the same time as Car M but a higher speed. This does not match our condition.
   - **Graph (C):** Car N has the same time as Car M but a lower speed. This does not match our condition.
   - **Graph (D):** Car N has half the time of Car M and a higher speed. This matches our condition.
   - **Graph (E):** Car N has the same time as Car M but a lower speed. This does not match our condition.

5. **Conclusion:**
   - The correct graph is (D), where Car N travels at twice the speed of Car M but takes half the time to travel the same distance.

The final answer is $\boxed{D}$