As for the applicable conditions of the law of conservation of mechanical energy, the correct one in the following statement is () A. When there are other external forces, as long as the combined external force is zero, the mechanical energy is conserved. C. when there are other external forces, as long as the work of the resultant force of other forces is zero, the mechanical energy is conserved. D. when the projectile flies in the air without resistance, it is only affected by the heavy force, so the mechanical energy before and after the explosion is conserved

As for the applicable conditions of the law of conservation of mechanical energy, the correct one in the following statement is () A. When there are other external forces, as long as the combined external force is zero, the mechanical energy is conserved. C. when there are other external forces, as long as the work of the resultant force of other forces is zero, the mechanical energy is conserved. D. when the projectile flies in the air without resistance, it is only affected by the heavy force, so the mechanical energy before and after the explosion is conserved

A. When only gravity or elastic force do work, mechanical energy is conserved, not only when gravity and elastic force act; so a is wrong; B, when only gravity or elastic force do work, combined external force is zero, mechanical energy is not necessarily conserved; for example, when the combined external force of an object falling vertically at a constant speed is zero, mechanical energy is not conserved, so B is wrong; C, the object is acted by other forces besides gravity and elastic force If the sum of work done by other forces is zero, the mechanical energy is conserved, so C is correct; if the projectile flies in the air without resistance, it is only affected by gravity, and the chemical energy is converted into mechanical energy in the process of projectile explosion; so the mechanical energy increases, so D is wrong; so C