Which of the following are characteristics of normal curves? select one or more: a. normal curves are symmetrical about a vertical line through the mean. b. normal curves have an area under the curve of 1. c. normal curves have an area of 1 between 3 standard deviations of the mean. d. normal curves approach the horizontal axis, but never touch or cross it. e. normal curves are positively skewed, symmetric, or negatively skewed. f. normal curves are bell-shaped with the highest point over the mean.