An arch carries the thrust of weight to its _____(1)______. With a _____(2)______, the horizontal part of the structure supports all the weight above it.
a. (1) center; (2) post-and-bearing
b. (1) base; (2) springing line
c. (1) keystone; (2) groin vault
d. (1) sides; (2) post-and-lintel