In Dealing with AIDS (Unit 4), the author thought that they were invincible until the word AIDS came into his life. Why did he think so?

A、Becausetheauthorisstrongenough.;
B、Becausehisfriend,David,didn’ttellhimaboutthedangerofAIDS.;
C、Becauseheistooyoungandenergetictorealizethatlifecouldbeveryfragile.;
D、BecausehereadaboutAIDSbuthasnevermetanyrealcaseinhislife.