Read the following passage carefully and then choose the best option to fit each space. Identify your choice by circling letter A, B, C or D on the answer sheet.

The 18^t^h-century battlefield was, compared with that of the 20^`^t^h-century, an intimate theatre, especially intimate in the (19)___ of the Revolution, which was usually small even by the standards of the day. Soldiers had to come to close quarters to kill; this fact reduced the mystery of the battle, though perhaps not its terrors. But at least the battle field lost some of its impersonality. In fact, in (20)___ to (with) the 20^1h-century combat, in which the enemy usually remains unseen and the source of incoming fire unknown, in the 18^1h-century battles the enemy could be seen and sometimes even (21)___ . (22)____ one's enemy may have arose a singular (23) ____of feeling uncommon in modern battles. Before the (24)___ occurred tension and anxiety (25) _____ up as the troops marched from their column into a line of attack. The ( 26) ____ of their movements was well understood by themselves and their enemies, who must have watched with feeling of dread and fascination. When the order came sending them (27) ____ rages, even madness replaced the attacker's (28) ____, while terror and desperation filled those who received the charge.

Điền vào số (20)

In the fourteenth century, ______ that glass coated with silver nitrate would turn yellow