Which of the following statements is true regarding the phospholipid bilayer in cell membranes?a) water and ions are unable to cross the bilayer due to the charges present in the head groupsb) water and ions are unable to cross the bilayer due to the hydrophobic tails of the phospholipid moleculesc) water and ions are encouraged to cross the bilayer by interacting with the charges present in the head groupsd) the molecules in the bilayer are fluid, and so the cell membrane is porous allowing the passage of ions and water across the cell membrane