I need to build a circuit that accepts a 4-bit number and turns on a LED (representing an error) if the given number is higher than 9.

As I understood, it should represent an "error display for a BCD counter". Since a BCD accepts a number of 4 bits (0-15) but it only counts from 0-9, I should turn on a LED that represents the presence of an error on the counter.

So, I followed the steps below:

1. Built the truth table;
2. Built the expression based on the truth table;
3. Built the Karnaugh Map;
4. Built the simplified expression from the Karnaugh Map;

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**1. Building the Truth Table**

*Note: the X column represents the output that shows an error (i.e. the LED)*

[![enter image description here][1]][1]


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**2. Building the expression based on the Truth Table**

So, based on the Truth Table, I got the following expressions:

*Note: consider the (!X) as a NOT X*

(D)(!C)(B)(!A) + (D)(!C)(B)(A) + (D)(C)(!B)(!A) + (D)(C)(!B)(A) + (D)(C)(B)(!A) + (D)(C)(B)(A)

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**3. Building the Karnaugh Map**

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/B5XgK.png
  [2]: https://i.sstatic.net/TSFIx.png

So, as you guys can see, I found two quartets.

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**4. Building the simplified expression from the Karnaugh Map**

As I found two quartets, this is the final expression I found:

(DC) + (DB)

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**The conclusion and the final question**

So, after all these steps, I tried to apply the Truth Table values into my simplified expression (ignoring the A column, as the simplified expression doesn't contain it). It seems to be correct after all.

**My question is:**

Let's suppose someone get close to me and says: 

*"I've built a BCD counter. It should only count from 0 to 9. If a number higher than 9 is given, then there is an error. I need YOU to build ANOTHER circuit. I'll give you the 4-bit number that entered on my circuit and then you need to show an error in case this number is higher than 9."*

Considering my final expression, *(DC) + (DB)*, should I just **ignore** the port A, since there is no A in my final simplified expression? Does it mean I can just leave A without being connected to any wire?