单项选择题
A．borrowed money B．waited for a good time to open his business C．developed an inadequate plan D．studied, prepared a plan, and trained himself
A．Glass is very useful because it has many unusual qualities. B．Light signals have changed the use of glass in industry. C．Glass fibers have reduced the cost of telephone communication. D．The use of glass fibers to carry telephone messages is an interesting new development.
A．sound signals B．light signals C．electrical signals D．any signals