若元素a, b, c, d, e, f依次进栈，允许进栈、退栈操作交替进行，但不允许连续三次进行退栈操作， 则不可能得到的出栈序列是（ ）。 A. d, c, e, b, f, a

B. c, b, d, a, e, f

C. b, c, a, e, f, d

D. a, f, e, d, c, b

Why is it uncommon for users to perform searches directly in database tables? a.)users are discouraged from interacting directly with tables because they might confuse tables with spreadsheets. b.) users are discouraged from interacting directly with tables because this may result in unintended changes to source data. c.)users do not have the technical skills required to perform searches directly in database tables. d.)users do not have the permissions required to perform searches directly in database tables.