help with this question please!!!1

The half-life of cesium-137 is 30 years. Suppose we have a 378-mg sample. Find the mass that remains after t years. a) 528 x 2-1/30 O b) 578 x 2-1/30 O c) 378 x 2-1/30 O d) 478 x 2-1/30 O e) 428 x 2-1/30