Among the items shipped in 1994 via truck, how many items were returned?
1994 refers to year(l_shipdate) = 1994; via truck refers to l_shipmode = 'TRUCK'; returned refers to l_returnflag = 'R'
SELECT COUNT(l_orderkey) FROM lineitem WHERE STRFTIME('%Y', l_shipdate) = '1994' AND l_returnflag = 'R' AND l_shipmode = 'TRUCK'