Which of the following is true of semiconductors?
a. exposing a crystal of a semiconductor to heat or light starts displacing valence electrons, which then move throughout the crystal.
b. a material is classified as a semiconductor if its resistance to the flow of electric current is too low to permit it to be called a conductor.
c. neither free electrons nor holes are considered charge carriers in a semiconductor.
d. gold and silver are important semiconductors

B. A material is classified as a semiconductor if its resistance to the flow of electric current is too low to permit it to be called a conductor.

Explanation:

A material is classified as a semiconductor if its resistance to the flow of electric current is too low to permit it to be called a conductor is true of semiconductors.

answer: at 24 february, 2014 the population gets twice of its initial population.

explanation:

since we have given that

the population for alpha city, t years after january 1, 2004 is given as

first we find out the initial population, i.e. t=0,

so, our quadratic equation becomes,

according to question, we have said that if the population has twice its initial population, then it becomes

so, time taken to reach this above mentioned population is given by

now, we know that time can't be negative so, we take t=9.149 years.

hence, at 24 february, 2014 the population gets twice of its initial population.