Rapid application development (RAD) systems purport to lighten the developer’s (46) by making it easy to program an application with simple intuitive steps.

RAD tools (47) the developer everything needed to construct a fully functional application quickly and easily. They make short work of modifying the developed product to (48) changes after the program is finished. Applications developed with RAD tools are constructed as they are envisioned. As an application’s real usefulness (49) , the developer armed with RAD tools should be able to (50) on new features with little fuss.

（50）是（）

A.beat

B.change

C.debug

D.tack

挤出成型也适合各种热固性塑料，且特别以适合石棉和碎布等作为填料的热固性塑料的成型。