单选题：Multiple Sclerosis（多发性硬化症） 1 Multiple Sclerosis (MS) is a di

Multiple Sclerosis（多发性硬化症）
1 Multiple Sclerosis (MS) is a disease in which the patient's immune（免疫的）system attacks the central nervous system. This can lead to numerous physical and mental symptoms, as the disease affects the transmission of electrical signals between the body and the brain. However, the human body, being a flexible, adaptable system, can compensate for some level of damage, so a person with MS can look and feel fine even though the disease is present.
2 MS patients can have one of two main varieties of the disease: the relapsing form （复发型）and the primary progressive form. In the relapsing form, the disease progresses in a series of jumps; at times it is in remission（减轻）.which means that a person's normal functions return for a period of time before the system goes into relapse and the disease again becomes more active. This is the most common form of MS; 80-90% of people have this form of the disease when they are first diagnosed. The relapse-remission cycle can continue for many years. Eventually, however, Ioss of physical and cognitive functions starts to take place and the remissions become less frequent.
3 In the primary progressive form of MS, there are no remissions and a continual but steady loss of physical and cognitive functions takes place. This condition affects about 10-15% of sufferers at diagnosis.
4 The expected course of the disease, or prognosis（预后）,depends on many variables: the subtype of the disease, the patient's individual characteristics and the initial symptoms. Life expectancy of patients, however, is often nearly the same as that of an unaffected person - provided that a reasonable standard of care is received. In some cases a near-normal life span is possible.
5 The cause of the disease is unclear; it seems that some people have a genetic Susceptibility（易感性）,which is triggered by some unknown environmental factor. Onset（发作）of the disease usually occurs in young adults between the ages of 20 and 40.It is more common in women than men; however, it has also been diagnosed in young children and in elderly people.
Paragraph 1___ ___ A．The relapsing form of MS
B．The cause of MS
C．The primary progressive form of MS
D．The definition of MS
E．The development of MS
F．The treatment for MS