1.A
1）across是个介词,swim across the river“游过那条河流”；
2）meaure 100 meters wide在此处做后置定语,修饰the river.
而measure作为“长度/宽度为...”时是不及物动词,表明the river和measure之间是主动关系,所以只能使用其现在分词measuring.
2.C
固定句型“How did it come/bring about that-从句?”意思是“.是怎么发生的?”
come back “回来”无此句型；
“it occur to sb that-从句”意思是“某人突然想起/想到...”；
get down “下来/使...下来”无此句型.