The answer here is D - bases tend to feel slippery. This is because they tend to dissolve oils on your skin, reducing the friction between your fingers or hands when you rub them together.
Atomic mass Ar => 39.948 a.m.u
39.948 g --------------- 6.02x10²³ atoms
?? g -------------------- 3.8x10²⁴ atoms
(3.8x10²⁴) x 39.948 / 6.02x10²³ => 250 g
hope this helps!
