During the early Middle Ages .a. Viking invasions caused the collapse of agriculture. b. developments such as the three-field system and the padded horse collar meant that agriculture actually improved from classical time. c. crop rotation was abandoned. d. crop rotation was developed.

b. developments such as the three-field system and the padded horse collar meant that agriculture actually improved from classical time.

Explanation:Early Middle Ages or Early Medieval Period , sometimes referred to as the Dark Ages , as lasting from the 5th or 6th

century to the 10th century.

In the 19th century the Early Middle Ages were often labelled the "Dark Ages" , a characterization based on the relative scarcity of literary and cultural output from this time.

The early middle Ages generally believed to have started with the fall of the Roman Empire in 476 and to have lasted about 1,000 years until about 1450. The beginning of the Middle Ages is called the Dark Ages because the great civilizations of Rome and Greece had been conquered.

a and b because with the other three, they simplify to something else.