That Ebbinghaus served as his own participant was problematic because:

a. his expectations might have influenced the results.

b. he could not manipulate an independent variable.

c. his studies were double-blind.

d. he didn't have any experimenter bias.

His expectation might have influenced the result

Explanation:

Hermann Ebbinghaus was the first psychologist who works on the systematical study of learning and memory carrying out by long exhaust experiments on himself. He argued that remembering is associated with the linking place, memory, time, cause, and effect. He tries to find out the effect of the association on memory, recording and mathematically see the pattern of memory to follow the aspect of the association. He tested different lengths and different leaning intervals, noting the speed of learning and forgetting. He found that he could remember meaningful materials such as poem ten times than his nonsense syllables. He also noted that more time the stimulus repeated, fewer times the times taken to reproduce the memorized information. Even the first few repetitions show the effect of memorization.