There are several principles that need to be considered when studying teratogenic effects. The................ (Choice: timing, type, amount, interaction) of teratogen influences the kind of outcome in the sense that different teratogens will have different negative effects on the growing organism. However, what also matters is the........... (Choices: timing, type, amount, interaction) of exposure in that there is some threshold that has to be crossed in order for that teratogen to have an effect such as ionizing radiation which does not have a huge effect in smaller doses but can lead to congenital malformations in larger doses. Congenital rubella (measles in the pregnant individual) can lead to malformations in the fetus particularly the earlier in pregnancy the rubella virus exposure occurs - demonstrating that................. (Choices: timing, type, amount, interaction)of exposure is another critical factor.