While typing, Helen has a habit of stopping to give her long and flowing hair a smooth.
A now and then B.over and over C.up and down D.here and there
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解析：now and then时而，不时； over and over反复，再三； up and down上上下下； here and there到处。句意是打字时，海伦有一个习惯，不时停下来捋一下她的头发。故选A。