The sum of the first digit and the second digit is the third digit, and so on. The horizontal line is $11+18=29$. 
Observe the characteristics of the numbers given and write the numbers underlined below.~(adapted from $$2006$$ Math kangaroo Problems, Level $$1-2$$, Question \#$$8$$)  $$3$$~ ~ $$4$$~ ~ $$7$$~ ~ $$11$$~ ~$18$~\uline{~~~~~~~~~~}~$47$~ ~$76$~ ~$123$~ $$\cdots$$ 
Options:
[a] $$21$$ 
[b] $$24$$ 
[c] $$27$$ 
[d] $$28$$ 
[e] $$29$$ 