scala map和flatMap

scala> val a  = Seq(1,2,3,6,4)
a: Seq[Int] = List(1, 2, 3, 6, 4)

scala>     val b = a.flatMap(f=>{
     |       try{
     |         Some(f/(f-1))
     |       }catch{
     |         case e:Exception=>None
     |       }
     |     })
b: Seq[Int] = List(2, 1, 1, 1)

scala>     val b = a.map(f=>{
     |       try{
     |         Some(f/(f-1))
     |       }catch{
     |         case e:Exception=>None
     |       }
     |     })
b: Seq[Option[Int]] = List(None, Some(2), Some(1), Some(1), Some(1))

scala>     val d = a.flatMap(f=>{
     |       try{
     |         f/(f-1)
     |       }catch{
     |         case e:Exception=>None
     |       }
     |     })
<console>:33: error: type mismatch;
 found   : Int
 required: scala.collection.GenTraversableOnce[?]
               f/(f-1)
                ^

scala>     val d = a.map(f=>{
     |       try{
     |         f/(f-1)
     |       }catch{
     |         case e:Exception=>None
     |       }
     |     })
d: Seq[Any] = List(None, 2, 1, 1, 1)