Read the following statements about cell structures. (i) A mitochondrion is bounded by two membranes. (ii) A chloroplast is bounded by two membranes. (iii) The nucleus is bounded by two membranes. (iv) A vacuole is bounded by two membranes. (v) A ribosome is bounded by two membranes. (vi) A centriole is bounded by two membranes. How many of the statements above are correct?