सकारात्मक रूप से त्वरित अधिगम वक्र (Positively Accelerated Learning Curve) में प्रगति का मुख्य कारण क्या है?

If we track the progress of a student learning drawing over a period of about 6 months, showing initial slow progress, followed by a sharp increase, and then a stabilization, the obtained learning curve is a/an: