Multiple choice
In a BB84 intercept-resend attack, Eve measures each qubit in a randomly chosen basis. With probability 1/2 she picks the correct (matching) basis and introduces no error; with probability 1/2 she picks the wrong basis and, after resending, causes Bob to get a wrong bit with probability 1/2. What is the resulting qubit error rate Eve induces (= P(correct)*0 + P(wrong)*1/2)?