Along with the letter was his promise _______ he would visit me this coming Christmas.  [ ]

A. that

B. which

C. what

D. whether

增大压强的两种方法是_________和__________；缝衣针的针头做得尖尖的，这是为了__________；缝厚衣服时手指常常带上顶针，是为了_______。