In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank out of the four alternatives._x005F_x000D_ ___________ it became difficult to empirically prove that children __________ better in private schools, the attack __________ a new weapon: per unit cost of learning outcomes. Most of the learning outcome researches almost _______________ fail to understand the entire ______________ of education in any depth and reduces it to learning of so­called 3Rs for economic purposes._x005F_x000D_ the attack __________ a new weapon_x005F_x000D_  _x005F_x000D_

• A. invents
• B. had invented
• C. is inventing
• D. invented