The change from thinking the continents are stationary to understanding that the outer layer of the ...

The change from thinking the continents are stationary to understanding that the outer layer of the earth moves slowly nearly all of the time occurred primarily because ________.
 
  A) we began to make global observations that required recognition that the continents and oceans had not always been in their current positions
  B) we had two world wars and learned a lot from them about the oceans that explained how continents moved
  C) we improved our surveying instruments significantly
  D) we got space travel and could see the motions