Besides a high voltage shock, what is another method to make e. coli competent to take up "naked" dna?
a. high concentrations of calcium ions followed by high temperature
b. high concentrations of calcium ions and several hours on ice
c. large amounts of dna added directly to a bacterial culture growing at 37c
d. high concentrations of minerals followed by high temperature
e. a high voltage shock is the only way to make e. coli competent