Select the topic sentence. [1] what is amnesia? [2] most of us think of a dramatic, total loss of memory brought on by a severe blow to the head. [3] amnesia is, however, a very subtle and sometimes sinister illness. [4] while amnesia can result from a head injury, overwork and worry can also lead to memory lapses. [5] in some cases, diagnosis is difficult because these lapses are intermittent, rather than constant. a.) 1 b.) 2 c.) 3 d.) 4 3.) 5