Three zones of varying propertie have been identified in the earth i.e. crust, mantle and core. (a) The area f change in idensity between sial and sima is known as Conrad discontinuity. (b) The area of sudden change in seismic waves velocity is known as Moho discontinuity. (c) The mantle - core boundary is determined by Gutenberg discontinuity. Which of the statements given above is/are correct ?

• A. (a) only
• B. (b) and (c) only
• C. (a) and (c) only
• D. (a), (b) and (c)