Baddeley & Hitch (1974) report an experiment in which participants were to perform a reasoning task of various difficulty ("A precedes B" T/F? "B does not precede A" T/F? etc.)

In addition to this primary task they were to repeat "the the the," repeatedly count from 1 to 6, or repeat a set of random digits. Performance was compared against single-task performance.
a. The dual-task demands affected each difficulty condition equally.
b. The dual-task demands of the verbal task did not affect reasoning performance.
c. The dual-task demands were particularly pronounced for the negative passive condition.
d. The dual-task demands were particularly pronounced when the second task was the random digit task, especially in the negative passive condition.

Correct: d