In copper, the valence band and conduction band overlap, allowing electrons to flow freely and making copper an excellent conductor. In contrast, in materials like silicon and germanium, there is a gap between the valence and conduction bands, making them semiconductors.
Match the following:
In the following, \( [x] \) denotes the greatest integer less than or equal to \( x \). 
Choose the correct answer from the options given below:
For x < 0:
f(x) = ex + ax
For x ≥ 0:
f(x) = b(x - 1)2