17 introductions: a plural noun is needed. The ‘e’ is dropped, the suffix ‘tion’ and an ‘s’ are added.

18 impolite: the negative prefix ‘im’ is added.

19 overcome: ‘over’ is added to ‘come’ to form a compound verb that collocates with ‘problem’.

20 observation: a singular noun is needed. The final ‘e’ is dropped and the suffix ‘ation’ added.

21 characteristics: a noun is required. The suffix ‘istics’ is added; it must be plural because two characteristics are mentioned (‘high foreheads’ and ‘narrow eyebrows’).

22 variety: a singular noun meaning ‘range’ is needed. The ‘y’ changes to ‘i’, and the suffix ‘ety’ is added.

23 visual: an adjective is needed. To form it, ‘ion’ changes to ‘ual’.

24 ensuring: the prefix ‘en’ is added to form the verb, after a preposition, the –ing form is required, but the final ‘e’ must first be dropped.

17 perilous: The context shows that an adjective is required before the noun `voyages'.

18 Unbelievable: Reading the whole sentence makes it clear that you need a word that suggests the information presented is hard, rather than easy, to believe.

19 risky: You need to add the suffix -y to make the adjective from `risk'.

20 Analysis: The verb `suggests' shows that you need the singular noun rather than the plural 'analyses'.

21 arrival: The definite article shows that a noun is required here.

22 settlement: The context makes it clear that you need a word that is referring to a place rather than a person (which would be 'settler).

23 plentiful: Remember to have only one '1' at the end of any adjective ending 'fur (apart from the word 'full', of course).

24 bravery: This is the abstract noun from 'brave'.