18. Based on the chemical formulae of various membrane phospholipids. Which statement about charge character is true? (A) PC, PE positive, PS, PI neutral in charge, (B) PC, PE neutral, PS, PI negative in charge, (C) PC, PE negative, PS, PI neutral in charge, (D) PC positive, PE negative, PS, PI neutral in charge, (E) none of above. Note P stands for phosphatidyl.