Explain why the 7 bit standard ascii code needs to have the highest bit set to a zero

ASCII (as opposed to ISO 8859) only uses 7 bits per character, and one byte per character. Byte numbers with the highest be set to 1 would have value 128-255, which is undefined by ASCII.

Some (old) serial communication protocols use only 7 bits per character. In such a protocol you wouldn't even be able to send the 8th bit because it isn't there.

It is always good to remember that in general a char is not a byte. Only in ISO 8859 this is the case. But in UTF-8 for instance, a char is 1, 2, 3 or 4 bytes.

Ibelieve the answer is: b.  use a different transition for every slide to keep the audience's attentiona. that would make it boringc. sound effects might be distracting and some people might find it annoyingi hope that : ) 

So is there any ways to explain this better?