Given b & d i marked a & c

31 Which of the following statement(s) is/are correct for a plot of ("Sr/6Sr) versus ("Rb/Sr) of a rock which has evolved in a closed system? (A) The slopes of the lines of evolution for the minerals in the rock are all equal and positive B) The slopes of the lines of evolution for the minerals in the rock are all equal and negative (C) The slope of the isochron is identical in direction to the slope of the lines of evolution for the minerals in the rock (D) The slope of the isochron is opposite in direction to the slope of the lines of evolution for the minerals in the rock