प्रत्येक वाक्य को पढ़ें और पता लगाएं कि क्या इसमें कोई व्याकरण संबंधी त्रुटि है और इसके लिए सही विकल्प चुनें।

• A
  His father told me
• B
  that though his son had
• C
  worked very hard,
• D
  but he had failed to make any mark in the examination.