We are very grateful for a number of friends and colleagues in encouraging us to start the work, preserve with it and finally to publish it.

First and foremost we would like to thank Mr. Pralhad Karki, Managing Director of Kantipur City College and Er. Rabi Shrestha, principal of Kantipur City College for their invaluable suggestions and encouragement in completing this book.

Secondly, we express our sincere thanks to Mr. Bikash Neupane, lecturer at Kantipur City College; for his moral support, showing confidence in our work and publishing this textbook.

Similarly, we would like to show our greatest appreciation to our colleagues Er. Ramesh Shrestha, Mr. Raju kattel, Er. Bishal Karki, Er. Subash Rajkarnikar, Er. Bimal Raj Pandey, Mr. Kiran Khanal, Mr. Bikash Banskota, Mr. Kishor Basnet, Mr. Shiba Ratna Tamrakar, Mr. Sudarsan Nepal, Mr. Ramesh Parajuli , Mr. Jeevan Budhathoki and Mr. Bhupal Man Budhathoki from Kantipur City College for their valuable guidance and advice.

Likewise, we wish to express our deep sense of gratitude to Mr. Jeevan Dangol, Managing Director of Modern Nepal College and Mr. Satya Narayan Shrestha, Principal of Modern Nepal College, who encouraged us to complete this book.

We would like to acknowledge with gratitude to our parents Mr. Prem Prasad Khatiwada, Ms. Bimala Khatiwada, Mr. Ishwari Pandey, Ms. Sabitra Pandey and our beloved family members Mr. Pawan Pandey, Ms. Eleeja Sharma, Ms. Sharmila Sharma, Mr. Bhim Prasad Poudel and Mr. Anmol Paudel for their understandings, endless love and encouragement when it was required.

Finally, we would like to gratefully acknowledge to Mr. Nabaraj Bajgain, Managing Director of Buddha Academic Enterprises Pvt. Ltd. for his enthusiasm, patience and support during our writing, as well as Mr. Dipendra Shrestha for computer works to publish this book.

Sunil Sharma and Saroj Pandey